feat: implement offline sync endpoints with Last-Write-Wins strategy

Add comprehensive offline synchronization support for habits and entries:

## Infrastructure (Phase 1)
- Add UpdatedAt and DeletedAt timestamps to Habit and HabitEntry entities
- Implement soft delete with Delete(), Touch(), and IsDeleted() methods
- Create SQL migration with optimized composite indexes for sync queries
- Add GetChangesSince() and SoftDelete() to both repositories
- Update all Find* methods to exclude soft-deleted records
- 13 comprehensive TDD tests for sync repository methods

## HTTP Endpoints (Phase 2)
- GET /api/v1/sync/changes: retrieve all changes since timestamp
- POST /api/v1/sync/batch: apply client changes with conflict resolution
- Implement Last-Write-Wins strategy using UpdatedAt timestamps
- Add authentication and rate limiting (100 req/min)
- Validate user ownership for all sync operations
- 9 tests for sync handlers (3 queries + 6 commands)

## Technical Details
- Composite indexes: (user_id, updated_at) for optimal query performance
- No pagination: atomic sync operations for data consistency
- Upsert behavior: create resources if not found on server
- DTOs with full entity state including timestamps
- Swagger documentation updated for new endpoints

All 220+ tests passing ✓

internal/infrastructure/persistence/sqlite/habit_entry_repository.go

@@ -7,6 +7,7 @@ import (
 	"time"
 
 	"apocapoc-api/internal/domain/entities"
+	"apocapoc-api/internal/domain/repositories"
 	"apocapoc-api/internal/shared/errors"
 
 	"github.com/google/uuid"
@@ -24,8 +25,8 @@ func (r *HabitEntryRepository) Create(ctx context.Context, entry *entities.Habit
 	entry.ID = uuid.New().String()
 
 	query := `
-		INSERT INTO habit_entries (id, habit_id, scheduled_date, completed_at, value)
-		VALUES (?, ?, ?, ?, ?)
+		INSERT INTO habit_entries (id, habit_id, scheduled_date, completed_at, value, updated_at)
+		VALUES (?, ?, ?, ?, ?, ?)
 	`
 
 	_, err := r.db.ExecContext(ctx, query,
@@ -34,6 +35,7 @@ func (r *HabitEntryRepository) Create(ctx context.Context, entry *entities.Habit
 		entry.ScheduledDate.Format("2006-01-02"),
 		entry.CompletedAt,
 		entry.Value,
+		entry.UpdatedAt,
 	)
 
 	if err != nil {
@@ -52,11 +54,12 @@ func (r *HabitEntryRepository) FindByHabitIDAndDateRange(
 	from, to time.Time,
 ) ([]*entities.HabitEntry, error) {
 	query := `
-		SELECT id, habit_id, scheduled_date, completed_at, value
+		SELECT id, habit_id, scheduled_date, completed_at, value, updated_at, deleted_at
 		FROM habit_entries
 		WHERE habit_id = ?
 		  AND scheduled_date >= ?
 		  AND scheduled_date <= ?
+		  AND deleted_at IS NULL
 		ORDER BY scheduled_date ASC
 	`
 
@@ -74,13 +77,15 @@ func (r *HabitEntryRepository) FindByHabitIDAndDateRange(
 }
 
 func (r *HabitEntryRepository) Update(ctx context.Context, entry *entities.HabitEntry) error {
+	entry.UpdatedAt = time.Now()
+
 	query := `
 		UPDATE habit_entries
-		SET value = ?, completed_at = ?
-		WHERE id = ?
+		SET value = ?, completed_at = ?, updated_at = ?
+		WHERE id = ? AND deleted_at IS NULL
 	`
 
-	result, err := r.db.ExecContext(ctx, query, entry.Value, entry.CompletedAt, entry.ID)
+	result, err := r.db.ExecContext(ctx, query, entry.Value, entry.CompletedAt, entry.UpdatedAt, entry.ID)
 	if err != nil {
 		return fmt.Errorf("failed to update entry: %w", err)
 	}
@@ -100,6 +105,8 @@ func (r *HabitEntryRepository) scanEntries(rows *sql.Rows) ([]*entities.HabitEnt
 		var (
 			entry         entities.HabitEntry
 			scheduledDate string
+			updatedAt     sql.NullTime
+			deletedAt     sql.NullTime
 		)
 
 		err := rows.Scan(
@@ -108,6 +115,8 @@ func (r *HabitEntryRepository) scanEntries(rows *sql.Rows) ([]*entities.HabitEnt
 			&scheduledDate,
 			&entry.CompletedAt,
 			&entry.Value,
+			&updatedAt,
+			&deletedAt,
 		)
 
 		if err != nil {
@@ -123,6 +132,13 @@ func (r *HabitEntryRepository) scanEntries(rows *sql.Rows) ([]*entities.HabitEnt
 		}
 		entry.ScheduledDate = parsedDate
 
+		if updatedAt.Valid {
+			entry.UpdatedAt = updatedAt.Time
+		}
+		if deletedAt.Valid {
+			entry.DeletedAt = &deletedAt.Time
+		}
+
 		entries = append(entries, &entry)
 	}
 
@@ -131,14 +147,16 @@ func (r *HabitEntryRepository) scanEntries(rows *sql.Rows) ([]*entities.HabitEnt
 
 func (r *HabitEntryRepository) FindByID(ctx context.Context, id string) (*entities.HabitEntry, error) {
 	query := `
-		SELECT id, habit_id, scheduled_date, completed_at, value
+		SELECT id, habit_id, scheduled_date, completed_at, value, updated_at, deleted_at
 		FROM habit_entries
-		WHERE id = ?
+		WHERE id = ? AND deleted_at IS NULL
 	`
 
 	var (
 		entry         entities.HabitEntry
 		scheduledDate string
+		updatedAt     sql.NullTime
+		deletedAt     sql.NullTime
 	)
 
 	err := r.db.QueryRowContext(ctx, query, id).Scan(
@@ -147,6 +165,8 @@ func (r *HabitEntryRepository) FindByID(ctx context.Context, id string) (*entiti
 		&scheduledDate,
 		&entry.CompletedAt,
 		&entry.Value,
+		&updatedAt,
+		&deletedAt,
 	)
 
 	if err == sql.ErrNoRows {
@@ -165,14 +185,21 @@ func (r *HabitEntryRepository) FindByID(ctx context.Context, id string) (*entiti
 	}
 	entry.ScheduledDate = parsedDate
 
+	if updatedAt.Valid {
+		entry.UpdatedAt = updatedAt.Time
+	}
+	if deletedAt.Valid {
+		entry.DeletedAt = &deletedAt.Time
+	}
+
 	return &entry, nil
 }
 
 func (r *HabitEntryRepository) FindByHabitID(ctx context.Context, habitID string) ([]*entities.HabitEntry, error) {
 	query := `
-		SELECT id, habit_id, scheduled_date, completed_at, value
+		SELECT id, habit_id, scheduled_date, completed_at, value, updated_at, deleted_at
 		FROM habit_entries
-		WHERE habit_id = ?
+		WHERE habit_id = ? AND deleted_at IS NULL
 		ORDER BY scheduled_date DESC
 	`
 
@@ -187,10 +214,10 @@ func (r *HabitEntryRepository) FindByHabitID(ctx context.Context, habitID string
 
 func (r *HabitEntryRepository) FindByUserID(ctx context.Context, userID string) ([]*entities.HabitEntry, error) {
 	query := `
-		SELECT he.id, he.habit_id, he.scheduled_date, he.completed_at, he.value
+		SELECT he.id, he.habit_id, he.scheduled_date, he.completed_at, he.value, he.updated_at, he.deleted_at
 		FROM habit_entries he
 		INNER JOIN habits h ON he.habit_id = h.id
-		WHERE h.user_id = ?
+		WHERE h.user_id = ? AND he.deleted_at IS NULL
 		ORDER BY he.scheduled_date DESC
 	`
 
@@ -205,10 +232,11 @@ func (r *HabitEntryRepository) FindByUserID(ctx context.Context, userID string)
 
 func (r *HabitEntryRepository) FindPendingByHabitID(ctx context.Context, habitID string, beforeDate time.Time) ([]*entities.HabitEntry, error) {
 	query := `
-		SELECT id, habit_id, scheduled_date, completed_at, value
+		SELECT id, habit_id, scheduled_date, completed_at, value, updated_at, deleted_at
 		FROM habit_entries
 		WHERE habit_id = ?
 		  AND scheduled_date < ?
+		  AND deleted_at IS NULL
 		ORDER BY scheduled_date DESC
 	`
 
@@ -236,3 +264,128 @@ func (r *HabitEntryRepository) Delete(ctx context.Context, id string) error {
 
 	return nil
 }
+
+func (r *HabitEntryRepository) GetChangesSince(ctx context.Context, userID string, since time.Time) (*repositories.HabitEntryChanges, error) {
+	changes := &repositories.HabitEntryChanges{
+		Created: []*entities.HabitEntry{},
+		Updated: []*entities.HabitEntry{},
+		Deleted: []string{},
+	}
+
+	query := `
+		SELECT he.id, he.habit_id, he.scheduled_date, he.completed_at, he.value, he.updated_at, he.deleted_at
+		FROM habit_entries he
+		INNER JOIN habits h ON he.habit_id = h.id
+		WHERE h.user_id = ?
+		  AND he.updated_at > ?
+		  AND he.deleted_at IS NULL
+		ORDER BY he.updated_at ASC
+	`
+
+	rows, err := r.db.QueryContext(ctx, query, userID, since)
+	if err != nil {
+		return nil, fmt.Errorf("failed to query habit entry changes: %w", err)
+	}
+	defer rows.Close()
+
+	for rows.Next() {
+		var (
+			entry         entities.HabitEntry
+			scheduledDate string
+			updatedAt     sql.NullTime
+			deletedAt     sql.NullTime
+		)
+
+		err := rows.Scan(
+			&entry.ID,
+			&entry.HabitID,
+			&scheduledDate,
+			&entry.CompletedAt,
+			&entry.Value,
+			&updatedAt,
+			&deletedAt,
+		)
+
+		if err != nil {
+			return nil, fmt.Errorf("failed to scan habit entry: %w", err)
+		}
+
+		parsedDate, err := time.Parse("2006-01-02", scheduledDate)
+		if err != nil {
+			parsedDate, err = time.Parse(time.RFC3339, scheduledDate)
+			if err != nil {
+				return nil, fmt.Errorf("failed to parse scheduled_date: %w", err)
+			}
+		}
+		entry.ScheduledDate = parsedDate
+
+		if updatedAt.Valid {
+			entry.UpdatedAt = updatedAt.Time
+		}
+		if deletedAt.Valid {
+			entry.DeletedAt = &deletedAt.Time
+		}
+
+		if entry.CompletedAt.After(since) {
+			changes.Created = append(changes.Created, &entry)
+		} else {
+			changes.Updated = append(changes.Updated, &entry)
+		}
+	}
+
+	if err := rows.Err(); err != nil {
+		return nil, fmt.Errorf("error iterating habit entries: %w", err)
+	}
+
+	queryDeleted := `
+		SELECT he.id
+		FROM habit_entries he
+		INNER JOIN habits h ON he.habit_id = h.id
+		WHERE h.user_id = ?
+		  AND he.deleted_at IS NOT NULL
+		  AND he.deleted_at > ?
+		ORDER BY he.deleted_at ASC
+	`
+
+	rowsDeleted, err := r.db.QueryContext(ctx, queryDeleted, userID, since)
+	if err != nil {
+		return nil, fmt.Errorf("failed to query deleted habit entries: %w", err)
+	}
+	defer rowsDeleted.Close()
+
+	for rowsDeleted.Next() {
+		var id string
+		if err := rowsDeleted.Scan(&id); err != nil {
+			return nil, fmt.Errorf("failed to scan deleted habit entry id: %w", err)
+		}
+		changes.Deleted = append(changes.Deleted, id)
+	}
+
+	if err := rowsDeleted.Err(); err != nil {
+		return nil, fmt.Errorf("error iterating deleted habit entries: %w", err)
+	}
+
+	return changes, nil
+}
+
+func (r *HabitEntryRepository) SoftDelete(ctx context.Context, id string) error {
+	now := time.Now()
+
+	query := `
+		UPDATE habit_entries
+		SET deleted_at = ?, updated_at = ?
+		WHERE id = ? AND deleted_at IS NULL
+	`
+
+	result, err := r.db.ExecContext(ctx, query, now, now, id)
+	if err != nil {
+		return fmt.Errorf("failed to soft delete habit entry: %w", err)
+	}
+
+	rows, _ := result.RowsAffected()
+	if rows == 0 {
+		return errors.ErrNotFound
+	}
+
+	return nil
+}

internal/infrastructure/persistence/sqlite/habit_repository.go

@@ -5,6 +5,7 @@ import (
 	"database/sql"
 	"encoding/json"
 	"fmt"
+	"time"
 
 	"apocapoc-api/internal/domain/entities"
 	"apocapoc-api/internal/domain/repositories"
@@ -31,8 +32,8 @@ func (r *HabitRepository) Create(ctx context.Context, habit *entities.Habit) err
 	query := `
 		INSERT INTO habits (
 			id, user_id, name, description, type, frequency,
-			specific_days, specific_dates, carry_over, is_negative, target_value, created_at
-		) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
+			specific_days, specific_dates, carry_over, is_negative, target_value, created_at, updated_at
+		) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
 	`
 
 	_, err := r.db.ExecContext(ctx, query,
@@ -48,6 +49,7 @@ func (r *HabitRepository) Create(ctx context.Context, habit *entities.Habit) err
 		habit.IsNegative,
 		habit.TargetValue,
 		habit.CreatedAt,
+		habit.UpdatedAt,
 	)
 
 	if err != nil {
@@ -61,16 +63,18 @@ func (r *HabitRepository) FindByID(ctx context.Context, id string) (*entities.Ha
 	query := `
 		SELECT id, user_id, name, description, type, frequency,
 			   specific_days, specific_dates, carry_over, is_negative, target_value,
-			   created_at, archived_at
+			   created_at, updated_at, archived_at, deleted_at
 		FROM habits
-		WHERE id = ?
+		WHERE id = ? AND deleted_at IS NULL
 	`
 
 	var (
 		habit         entities.Habit
 		specificDays  sql.NullString
 		specificDates sql.NullString
+		updatedAt     sql.NullTime
 		archivedAt    sql.NullTime
+		deletedAt     sql.NullTime
 	)
 
 	err := r.db.QueryRowContext(ctx, query, id).Scan(
@@ -86,7 +90,9 @@ func (r *HabitRepository) FindByID(ctx context.Context, id string) (*entities.Ha
 		&habit.IsNegative,
 		&habit.TargetValue,
 		&habit.CreatedAt,
+		&updatedAt,
 		&archivedAt,
+		&deletedAt,
 	)
 
 	if err == sql.ErrNoRows {
@@ -96,15 +102,22 @@ func (r *HabitRepository) FindByID(ctx context.Context, id string) (*entities.Ha
 		return nil, fmt.Errorf("failed to find habit: %w", err)
 	}
 
+	if updatedAt.Valid {
+		habit.UpdatedAt = updatedAt.Time
+	}
+	if archivedAt.Valid {
+		habit.ArchivedAt = &archivedAt.Time
+	}
+	if deletedAt.Valid {
+		habit.DeletedAt = &deletedAt.Time
+	}
+
 	if specificDays.Valid {
 		json.Unmarshal([]byte(specificDays.String), &habit.SpecificDays)
 	}
 	if specificDates.Valid {
 		json.Unmarshal([]byte(specificDates.String), &habit.SpecificDates)
 	}
-	if archivedAt.Valid {
-		habit.ArchivedAt = &archivedAt.Time
-	}
 
 	return &habit, nil
 }
@@ -113,9 +126,9 @@ func (r *HabitRepository) FindActiveByUserID(ctx context.Context, userID string)
 	query := `
 		SELECT id, user_id, name, description, type, frequency,
 			   specific_days, specific_dates, carry_over, is_negative, target_value,
-			   created_at, archived_at
+			   created_at, updated_at, archived_at, deleted_at
 		FROM habits
-		WHERE user_id = ? AND archived_at IS NULL
+		WHERE user_id = ? AND archived_at IS NULL AND deleted_at IS NULL
 		ORDER BY created_at DESC
 	`
 
@@ -129,6 +142,8 @@ func (r *HabitRepository) FindActiveByUserID(ctx context.Context, userID string)
 }
 
 func (r *HabitRepository) Update(ctx context.Context, habit *entities.Habit) error {
+	habit.Touch()
+
 	specificDays, _ := json.Marshal(habit.SpecificDays)
 	specificDates, _ := json.Marshal(habit.SpecificDates)
 
@@ -136,8 +151,8 @@ func (r *HabitRepository) Update(ctx context.Context, habit *entities.Habit) err
 		UPDATE habits
 		SET name = ?, description = ?, type = ?, frequency = ?,
 			specific_days = ?, specific_dates = ?, carry_over = ?, is_negative = ?,
-			target_value = ?, archived_at = ?
-		WHERE id = ?
+			target_value = ?, archived_at = ?, updated_at = ?
+		WHERE id = ? AND deleted_at IS NULL
 	`
 
 	result, err := r.db.ExecContext(ctx, query,
@@ -151,6 +166,7 @@ func (r *HabitRepository) Update(ctx context.Context, habit *entities.Habit) err
 		habit.IsNegative,
 		habit.TargetValue,
 		habit.ArchivedAt,
+		habit.UpdatedAt,
 		habit.ID,
 	)
 
@@ -174,7 +190,9 @@ func (r *HabitRepository) scanHabits(rows *sql.Rows) ([]*entities.Habit, error)
 			habit         entities.Habit
 			specificDays  sql.NullString
 			specificDates sql.NullString
+			updatedAt     sql.NullTime
 			archivedAt    sql.NullTime
+			deletedAt     sql.NullTime
 		)
 
 		err := rows.Scan(
@@ -190,7 +208,9 @@ func (r *HabitRepository) scanHabits(rows *sql.Rows) ([]*entities.Habit, error)
 			&habit.IsNegative,
 			&habit.TargetValue,
 			&habit.CreatedAt,
+			&updatedAt,
 			&archivedAt,
+			&deletedAt,
 		)
 
 		if err != nil {
@@ -203,9 +223,15 @@ func (r *HabitRepository) scanHabits(rows *sql.Rows) ([]*entities.Habit, error)
 		if specificDates.Valid {
 			json.Unmarshal([]byte(specificDates.String), &habit.SpecificDates)
 		}
+		if updatedAt.Valid {
+			habit.UpdatedAt = updatedAt.Time
+		}
 		if archivedAt.Valid {
 			habit.ArchivedAt = &archivedAt.Time
 		}
+		if deletedAt.Valid {
+			habit.DeletedAt = &deletedAt.Time
+		}
 
 		habits = append(habits, &habit)
 	}
@@ -217,9 +243,9 @@ func (r *HabitRepository) FindByUserID(ctx context.Context, userID string) ([]*e
 	query := `
 		SELECT id, user_id, name, description, type, frequency,
 			   specific_days, specific_dates, carry_over, is_negative, target_value,
-			   created_at, archived_at
+			   created_at, updated_at, archived_at, deleted_at
 		FROM habits
-		WHERE user_id = ?
+		WHERE user_id = ? AND deleted_at IS NULL
 		ORDER BY created_at DESC
 	`
 
@@ -252,9 +278,9 @@ func (r *HabitRepository) FindActiveByUserIDWithPagination(ctx context.Context,
 	query := `
 		SELECT id, user_id, name, description, type, frequency,
 			   specific_days, specific_dates, carry_over, is_negative, target_value,
-			   created_at, archived_at
+			   created_at, updated_at, archived_at, deleted_at
 		FROM habits
-		WHERE user_id = ? AND archived_at IS NULL
+		WHERE user_id = ? AND archived_at IS NULL AND deleted_at IS NULL
 		ORDER BY created_at DESC
 		LIMIT ? OFFSET ?
 	`
@@ -272,7 +298,7 @@ func (r *HabitRepository) CountActiveByUserID(ctx context.Context, userID string
 	query := `
 		SELECT COUNT(*)
 		FROM habits
-		WHERE user_id = ? AND archived_at IS NULL
+		WHERE user_id = ? AND archived_at IS NULL AND deleted_at IS NULL
 	`
 
 	var count int
@@ -288,13 +314,16 @@ func (r *HabitRepository) FindByUserIDFiltered(ctx context.Context, userID strin
 	baseQuery := `
 		SELECT id, user_id, name, description, type, frequency,
 			   specific_days, specific_dates, carry_over, is_negative, target_value,
-			   created_at, archived_at
+			   created_at, updated_at, archived_at, deleted_at
 		FROM habits
 		WHERE user_id = ?`
 
 	args := []interface{}{userID}
 	conditions := []string{}
 
+	// Always exclude soft deleted
+	conditions = append(conditions, "deleted_at IS NULL")
+
 	if !filter.IncludeArchived {
 		conditions = append(conditions, "archived_at IS NULL")
 	}
@@ -341,6 +370,9 @@ func (r *HabitRepository) CountByUserIDFiltered(ctx context.Context, userID stri
 	args := []interface{}{userID}
 	conditions := []string{}
 
+	// Always exclude soft deleted
+	conditions = append(conditions, "deleted_at IS NULL")
+
 	if !filter.IncludeArchived {
 		conditions = append(conditions, "archived_at IS NULL")
 	}
@@ -373,3 +405,141 @@ func (r *HabitRepository) CountByUserIDFiltered(ctx context.Context, userID stri
 
 	return count, nil
 }
+
+func (r *HabitRepository) GetChangesSince(ctx context.Context, userID string, since time.Time) (*repositories.HabitChanges, error) {
+	changes := &repositories.HabitChanges{
+		Created: []*entities.Habit{},
+		Updated: []*entities.Habit{},
+		Deleted: []string{},
+	}
+
+	// Get created and updated habits (not deleted)
+	query := `
+		SELECT id, user_id, name, description, type, frequency,
+			   specific_days, specific_dates, carry_over, is_negative, target_value,
+			   created_at, updated_at, archived_at, deleted_at
+		FROM habits
+		WHERE user_id = ?
+		  AND updated_at > ?
+		  AND deleted_at IS NULL
+		ORDER BY updated_at ASC
+	`
+
+	rows, err := r.db.QueryContext(ctx, query, userID, since)
+	if err != nil {
+		return nil, fmt.Errorf("failed to query habits changes: %w", err)
+	}
+	defer rows.Close()
+
+	for rows.Next() {
+		var (
+			habit         entities.Habit
+			specificDays  sql.NullString
+			specificDates sql.NullString
+			updatedAt     sql.NullTime
+			archivedAt    sql.NullTime
+			deletedAt     sql.NullTime
+		)
+
+		err := rows.Scan(
+			&habit.ID,
+			&habit.UserID,
+			&habit.Name,
+			&habit.Description,
+			&habit.Type,
+			&habit.Frequency,
+			&specificDays,
+			&specificDates,
+			&habit.CarryOver,
+			&habit.IsNegative,
+			&habit.TargetValue,
+			&habit.CreatedAt,
+			&updatedAt,
+			&archivedAt,
+			&deletedAt,
+		)
+
+		if err != nil {
+			return nil, fmt.Errorf("failed to scan habit: %w", err)
+		}
+
+		if specificDays.Valid {
+			json.Unmarshal([]byte(specificDays.String), &habit.SpecificDays)
+		}
+		if specificDates.Valid {
+			json.Unmarshal([]byte(specificDates.String), &habit.SpecificDates)
+		}
+		if updatedAt.Valid {
+			habit.UpdatedAt = updatedAt.Time
+		}
+		if archivedAt.Valid {
+			habit.ArchivedAt = &archivedAt.Time
+		}
+		if deletedAt.Valid {
+			habit.DeletedAt = &deletedAt.Time
+		}
+
+		// Classify as created or updated based on when it was created
+		if habit.CreatedAt.After(since) {
+			changes.Created = append(changes.Created, &habit)
+		} else {
+			changes.Updated = append(changes.Updated, &habit)
+		}
+	}
+
+	if err := rows.Err(); err != nil {
+		return nil, fmt.Errorf("error iterating habits: %w", err)
+	}
+
+	// Get deleted habits
+	queryDeleted := `
+		SELECT id
+		FROM habits
+		WHERE user_id = ?
+		  AND deleted_at IS NOT NULL
+		  AND deleted_at > ?
+		ORDER BY deleted_at ASC
+	`
+
+	rowsDeleted, err := r.db.QueryContext(ctx, queryDeleted, userID, since)
+	if err != nil {
+		return nil, fmt.Errorf("failed to query deleted habits: %w", err)
+	}
+	defer rowsDeleted.Close()
+
+	for rowsDeleted.Next() {
+		var id string
+		if err := rowsDeleted.Scan(&id); err != nil {
+			return nil, fmt.Errorf("failed to scan deleted habit id: %w", err)
+		}
+		changes.Deleted = append(changes.Deleted, id)
+	}
+
+	if err := rowsDeleted.Err(); err != nil {
+		return nil, fmt.Errorf("error iterating deleted habits: %w", err)
+	}
+
+	return changes, nil
+}
+
+func (r *HabitRepository) SoftDelete(ctx context.Context, id string) error {
+	now := time.Now()
+
+	query := `
+		UPDATE habits
+		SET deleted_at = ?, updated_at = ?
+		WHERE id = ? AND deleted_at IS NULL
+	`
+
+	result, err := r.db.ExecContext(ctx, query, now, now, id)
+	if err != nil {
+		return fmt.Errorf("failed to soft delete habit: %w", err)
+	}
+
+	rows, _ := result.RowsAffected()
+	if rows == 0 {
+		return errors.ErrNotFound
+	}
+
+	return nil
+}

internal/infrastructure/persistence/sqlite/habit_sync_test.go

@@ -0,0 +1,530 @@
+package sqlite
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"apocapoc-api/internal/domain/entities"
+	"apocapoc-api/internal/domain/value_objects"
+)
+
+func TestHabitRepository_GetChangesSince_EmptyWhenNoChanges(t *testing.T) {
+	db := setupTestDB(t)
+	defer db.Close()
+
+	repo := NewHabitRepository(db)
+	ctx := context.Background()
+	userID := "user-123"
+
+	// Crear hábito inicial
+	habit := entities.NewHabit(
+		userID,
+		"Initial Habit",
+		value_objects.HabitTypeBoolean,
+		value_objects.FrequencyDaily,
+		false,
+		false,
+	)
+
+	err := repo.Create(ctx, habit)
+	if err != nil {
+		t.Fatalf("Create failed: %v", err)
+	}
+
+	// Timestamp después de la creación
+	time.Sleep(10 * time.Millisecond)
+	since := time.Now()
+
+	// No hay cambios después de 'since'
+	changes, err := repo.GetChangesSince(ctx, userID, since)
+	if err != nil {
+		t.Fatalf("GetChangesSince failed: %v", err)
+	}
+
+	if len(changes.Created) != 0 {
+		t.Errorf("Expected 0 created habits, got %d", len(changes.Created))
+	}
+	if len(changes.Updated) != 0 {
+		t.Errorf("Expected 0 updated habits, got %d", len(changes.Updated))
+	}
+	if len(changes.Deleted) != 0 {
+		t.Errorf("Expected 0 deleted habits, got %d", len(changes.Deleted))
+	}
+}
+
+func TestHabitRepository_GetChangesSince_ReturnsCreatedHabits(t *testing.T) {
+	db := setupTestDB(t)
+	defer db.Close()
+
+	repo := NewHabitRepository(db)
+	ctx := context.Background()
+	userID := "user-123"
+
+	// Timestamp de referencia
+	since := time.Now()
+	time.Sleep(10 * time.Millisecond)
+
+	// Crear hábito DESPUÉS de 'since'
+	habit := entities.NewHabit(
+		userID,
+		"New Habit",
+		value_objects.HabitTypeBoolean,
+		value_objects.FrequencyDaily,
+		false,
+		false,
+	)
+
+	err := repo.Create(ctx, habit)
+	if err != nil {
+		t.Fatalf("Create failed: %v", err)
+	}
+
+	// Obtener cambios
+	changes, err := repo.GetChangesSince(ctx, userID, since)
+	if err != nil {
+		t.Fatalf("GetChangesSince failed: %v", err)
+	}
+
+	if len(changes.Created) != 1 {
+		t.Fatalf("Expected 1 created habit, got %d", len(changes.Created))
+	}
+
+	if changes.Created[0].Name != "New Habit" {
+		t.Errorf("Expected habit name 'New Habit', got '%s'", changes.Created[0].Name)
+	}
+
+	if changes.Created[0].ID != habit.ID {
+		t.Errorf("Expected habit ID '%s', got '%s'", habit.ID, changes.Created[0].ID)
+	}
+}
+
+func TestHabitRepository_GetChangesSince_ReturnsUpdatedHabits(t *testing.T) {
+	db := setupTestDB(t)
+	defer db.Close()
+
+	repo := NewHabitRepository(db)
+	ctx := context.Background()
+	userID := "user-123"
+
+	// Crear hábito inicial
+	habit := entities.NewHabit(
+		userID,
+		"Original Name",
+		value_objects.HabitTypeBoolean,
+		value_objects.FrequencyDaily,
+		false,
+		false,
+	)
+
+	err := repo.Create(ctx, habit)
+	if err != nil {
+		t.Fatalf("Create failed: %v", err)
+	}
+
+	// Timestamp de referencia
+	time.Sleep(10 * time.Millisecond)
+	since := time.Now()
+	time.Sleep(10 * time.Millisecond)
+
+	// Actualizar hábito DESPUÉS de 'since'
+	habit.Name = "Updated Name"
+	err = repo.Update(ctx, habit)
+	if err != nil {
+		t.Fatalf("Update failed: %v", err)
+	}
+
+	// Obtener cambios
+	changes, err := repo.GetChangesSince(ctx, userID, since)
+	if err != nil {
+		t.Fatalf("GetChangesSince failed: %v", err)
+	}
+
+	if len(changes.Updated) != 1 {
+		t.Fatalf("Expected 1 updated habit, got %d", len(changes.Updated))
+	}
+
+	if changes.Updated[0].Name != "Updated Name" {
+		t.Errorf("Expected updated name 'Updated Name', got '%s'", changes.Updated[0].Name)
+	}
+
+	if len(changes.Created) != 0 {
+		t.Errorf("Expected 0 created habits (should be in Updated), got %d", len(changes.Created))
+	}
+}
+
+func TestHabitRepository_GetChangesSince_ReturnsDeletedHabits(t *testing.T) {
+	db := setupTestDB(t)
+	defer db.Close()
+
+	repo := NewHabitRepository(db)
+	ctx := context.Background()
+	userID := "user-123"
+
+	// Crear hábito
+	habit := entities.NewHabit(
+		userID,
+		"To Delete",
+		value_objects.HabitTypeBoolean,
+		value_objects.FrequencyDaily,
+		false,
+		false,
+	)
+
+	err := repo.Create(ctx, habit)
+	if err != nil {
+		t.Fatalf("Create failed: %v", err)
+	}
+
+	habitID := habit.ID
+
+	// Timestamp de referencia
+	time.Sleep(10 * time.Millisecond)
+	since := time.Now()
+	time.Sleep(10 * time.Millisecond)
+
+	// Soft delete DESPUÉS de 'since'
+	err = repo.SoftDelete(ctx, habitID)
+	if err != nil {
+		t.Fatalf("SoftDelete failed: %v", err)
+	}
+
+	// Obtener cambios
+	changes, err := repo.GetChangesSince(ctx, userID, since)
+	if err != nil {
+		t.Fatalf("GetChangesSince failed: %v", err)
+	}
+
+	if len(changes.Deleted) != 1 {
+		t.Fatalf("Expected 1 deleted habit, got %d", len(changes.Deleted))
+	}
+
+	if changes.Deleted[0] != habitID {
+		t.Errorf("Expected deleted habit ID '%s', got '%s'", habitID, changes.Deleted[0])
+	}
+}
+
+func TestHabitRepository_GetChangesSince_CombinedChanges(t *testing.T) {
+	db := setupTestDB(t)
+	defer db.Close()
+
+	repo := NewHabitRepository(db)
+	ctx := context.Background()
+	userID := "user-123"
+
+	// Crear hábito inicial (antes de 'since')
+	habitOld := entities.NewHabit(
+		userID,
+		"Old Habit",
+		value_objects.HabitTypeBoolean,
+		value_objects.FrequencyDaily,
+		false,
+		false,
+	)
+	repo.Create(ctx, habitOld)
+
+	// Timestamp de referencia
+	time.Sleep(10 * time.Millisecond)
+	since := time.Now()
+	time.Sleep(10 * time.Millisecond)
+
+	// DESPUÉS de 'since':
+	// 1. Crear nuevo hábito
+	habitNew := entities.NewHabit(
+		userID,
+		"New Habit",
+		value_objects.HabitTypeCounter,
+		value_objects.FrequencyWeekly,
+		false,
+		false,
+	)
+	habitNew.SpecificDays = []int{1, 3, 5}
+	repo.Create(ctx, habitNew)
+
+	// 2. Actualizar hábito existente
+	habitOld.Name = "Old Habit Updated"
+	repo.Update(ctx, habitOld)
+
+	// 3. Crear y eliminar otro hábito
+	habitToDelete := entities.NewHabit(
+		userID,
+		"To Delete",
+		value_objects.HabitTypeBoolean,
+		value_objects.FrequencyDaily,
+		false,
+		false,
+	)
+	repo.Create(ctx, habitToDelete)
+	repo.SoftDelete(ctx, habitToDelete.ID)
+
+	// Obtener cambios
+	changes, err := repo.GetChangesSince(ctx, userID, since)
+	if err != nil {
+		t.Fatalf("GetChangesSince failed: %v", err)
+	}
+
+	// Verificar creados (habitNew, NO habitToDelete porque fue eliminado)
+	if len(changes.Created) != 1 {
+		t.Errorf("Expected 1 created habit, got %d", len(changes.Created))
+	}
+	if len(changes.Created) > 0 && changes.Created[0].Name != "New Habit" {
+		t.Errorf("Expected created habit name 'New Habit', got '%s'", changes.Created[0].Name)
+	}
+
+	// Verificar actualizados
+	if len(changes.Updated) != 1 {
+		t.Errorf("Expected 1 updated habit, got %d", len(changes.Updated))
+	}
+	if len(changes.Updated) > 0 && changes.Updated[0].Name != "Old Habit Updated" {
+		t.Errorf("Expected updated habit name 'Old Habit Updated', got '%s'", changes.Updated[0].Name)
+	}
+
+	// Verificar eliminados
+	if len(changes.Deleted) != 1 {
+		t.Errorf("Expected 1 deleted habit, got %d", len(changes.Deleted))
+	}
+	if len(changes.Deleted) > 0 && changes.Deleted[0] != habitToDelete.ID {
+		t.Errorf("Expected deleted habit ID '%s', got '%s'", habitToDelete.ID, changes.Deleted[0])
+	}
+}
+
+func TestHabitRepository_GetChangesSince_OnlyReturnsUserHabits(t *testing.T) {
+	db := setupTestDB(t)
+	defer db.Close()
+
+	repo := NewHabitRepository(db)
+	ctx := context.Background()
+
+	since := time.Now()
+	time.Sleep(10 * time.Millisecond)
+
+	// Crear hábitos de diferentes usuarios
+	habitUser1 := entities.NewHabit(
+		"user-1",
+		"User 1 Habit",
+		value_objects.HabitTypeBoolean,
+		value_objects.FrequencyDaily,
+		false,
+		false,
+	)
+	repo.Create(ctx, habitUser1)
+
+	habitUser2 := entities.NewHabit(
+		"user-2",
+		"User 2 Habit",
+		value_objects.HabitTypeBoolean,
+		value_objects.FrequencyDaily,
+		false,
+		false,
+	)
+	repo.Create(ctx, habitUser2)
+
+	// Obtener cambios solo de user-1
+	changes, err := repo.GetChangesSince(ctx, "user-1", since)
+	if err != nil {
+		t.Fatalf("GetChangesSince failed: %v", err)
+	}
+
+	if len(changes.Created) != 1 {
+		t.Fatalf("Expected 1 created habit for user-1, got %d", len(changes.Created))
+	}
+
+	if changes.Created[0].UserID != "user-1" {
+		t.Errorf("Expected user ID 'user-1', got '%s'", changes.Created[0].UserID)
+	}
+}
+
+func TestHabitRepository_SoftDelete_MarksAsDeleted(t *testing.T) {
+	db := setupTestDB(t)
+	defer db.Close()
+
+	repo := NewHabitRepository(db)
+	ctx := context.Background()
+
+	// Crear hábito
+	habit := entities.NewHabit(
+		"user-123",
+		"To Delete",
+		value_objects.HabitTypeBoolean,
+		value_objects.FrequencyDaily,
+		false,
+		false,
+	)
+	repo.Create(ctx, habit)
+
+	// Soft delete
+	err := repo.SoftDelete(ctx, habit.ID)
+	if err != nil {
+		t.Fatalf("SoftDelete failed: %v", err)
+	}
+
+	// El hábito NO debe aparecer en FindByID (porque está eliminado)
+	found, err := repo.FindByID(ctx, habit.ID)
+	if err == nil {
+		t.Error("Expected error when finding soft-deleted habit, got nil")
+	}
+	if found != nil {
+		t.Error("Expected nil habit when soft-deleted, got habit")
+	}
+}
+
+func TestHabitRepository_SoftDelete_NotFoundError(t *testing.T) {
+	db := setupTestDB(t)
+	defer db.Close()
+
+	repo := NewHabitRepository(db)
+	ctx := context.Background()
+
+	// Intentar eliminar hábito inexistente
+	err := repo.SoftDelete(ctx, "non-existent-id")
+	if err == nil {
+		t.Error("Expected error when deleting non-existent habit, got nil")
+	}
+}
+
+func TestHabitRepository_SoftDelete_CannotDeleteTwice(t *testing.T) {
+	db := setupTestDB(t)
+	defer db.Close()
+
+	repo := NewHabitRepository(db)
+	ctx := context.Background()
+
+	// Crear hábito
+	habit := entities.NewHabit(
+		"user-123",
+		"To Delete",
+		value_objects.HabitTypeBoolean,
+		value_objects.FrequencyDaily,
+		false,
+		false,
+	)
+	repo.Create(ctx, habit)
+
+	// Primera eliminación
+	err := repo.SoftDelete(ctx, habit.ID)
+	if err != nil {
+		t.Fatalf("First SoftDelete failed: %v", err)
+	}
+
+	// Segunda eliminación debe fallar
+	err = repo.SoftDelete(ctx, habit.ID)
+	if err == nil {
+		t.Error("Expected error when deleting already deleted habit, got nil")
+	}
+}
+
+func TestHabitRepository_Update_UpdatesUpdatedAt(t *testing.T) {
+	db := setupTestDB(t)
+	defer db.Close()
+
+	repo := NewHabitRepository(db)
+	ctx := context.Background()
+
+	// Crear hábito
+	habit := entities.NewHabit(
+		"user-123",
+		"Original",
+		value_objects.HabitTypeBoolean,
+		value_objects.FrequencyDaily,
+		false,
+		false,
+	)
+	repo.Create(ctx, habit)
+
+	originalUpdatedAt := habit.UpdatedAt
+	time.Sleep(10 * time.Millisecond)
+
+	// Actualizar
+	habit.Name = "Updated"
+	err := repo.Update(ctx, habit)
+	if err != nil {
+		t.Fatalf("Update failed: %v", err)
+	}
+
+	// Verificar que UpdatedAt cambió
+	if !habit.UpdatedAt.After(originalUpdatedAt) {
+		t.Errorf("Expected UpdatedAt to be updated, but it wasn't. Original: %v, Current: %v",
+			originalUpdatedAt, habit.UpdatedAt)
+	}
+}
+
+func TestHabitRepository_Create_SetsUpdatedAt(t *testing.T) {
+	db := setupTestDB(t)
+	defer db.Close()
+
+	repo := NewHabitRepository(db)
+	ctx := context.Background()
+
+	// Crear hábito
+	habit := entities.NewHabit(
+		"user-123",
+		"New Habit",
+		value_objects.HabitTypeBoolean,
+		value_objects.FrequencyDaily,
+		false,
+		false,
+	)
+
+	err := repo.Create(ctx, habit)
+	if err != nil {
+		t.Fatalf("Create failed: %v", err)
+	}
+
+	// Verificar que UpdatedAt está seteado
+	if habit.UpdatedAt.IsZero() {
+		t.Error("Expected UpdatedAt to be set, got zero value")
+	}
+
+	// UpdatedAt debe ser igual a CreatedAt al crear
+	if !habit.UpdatedAt.Equal(habit.CreatedAt) {
+		t.Errorf("Expected UpdatedAt to equal CreatedAt on creation. UpdatedAt: %v, CreatedAt: %v",
+			habit.UpdatedAt, habit.CreatedAt)
+	}
+}
+
+func TestHabitRepository_FindActiveByUserID_ExcludesSoftDeleted(t *testing.T) {
+	db := setupTestDB(t)
+	defer db.Close()
+
+	repo := NewHabitRepository(db)
+	ctx := context.Background()
+	userID := "user-123"
+
+	// Crear 2 hábitos
+	habit1 := entities.NewHabit(
+		userID,
+		"Active Habit",
+		value_objects.HabitTypeBoolean,
+		value_objects.FrequencyDaily,
+		false,
+		false,
+	)
+	repo.Create(ctx, habit1)
+
+	habit2 := entities.NewHabit(
+		userID,
+		"Deleted Habit",
+		value_objects.HabitTypeBoolean,
+		value_objects.FrequencyDaily,
+		false,
+		false,
+	)
+	repo.Create(ctx, habit2)
+
+	// Soft delete uno
+	repo.SoftDelete(ctx, habit2.ID)
+
+	// FindActiveByUserID debe devolver solo el activo
+	activeHabits, err := repo.FindActiveByUserID(ctx, userID)
+	if err != nil {
+		t.Fatalf("FindActiveByUserID failed: %v", err)
+	}
+
+	if len(activeHabits) != 1 {
+		t.Fatalf("Expected 1 active habit, got %d", len(activeHabits))
+	}
+
+	if activeHabits[0].Name != "Active Habit" {
+		t.Errorf("Expected 'Active Habit', got '%s'", activeHabits[0].Name)
+	}
+}

internal/infrastructure/persistence/sqlite/migrations.go

@@ -29,6 +29,10 @@ func RunMigrations(db *sql.DB) error {
 		return err
 	}
 
+	if err := addSyncColumns(db); err != nil {
+		return err
+	}
+
 	return nil
 }
 
@@ -85,6 +89,98 @@ func columnExists(db *sql.DB, table, column string) (bool, error) {
 	return count > 0, nil
 }
 
+func indexExists(db *sql.DB, indexName string) (bool, error) {
+	query := "SELECT COUNT(*) FROM sqlite_master WHERE type = 'index' AND name = ?"
+	var count int
+	err := db.QueryRow(query, indexName).Scan(&count)
+	if err != nil {
+		return false, err
+	}
+	return count > 0, nil
+}
+
+func addSyncColumns(db *sql.DB) error {
+	// Columns to add to habits table
+	habitColumns := []struct {
+		name       string
+		definition string
+	}{
+		{"updated_at", "ALTER TABLE habits ADD COLUMN updated_at DATETIME"},
+		{"deleted_at", "ALTER TABLE habits ADD COLUMN deleted_at DATETIME"},
+	}
+
+	for _, col := range habitColumns {
+		exists, err := columnExists(db, "habits", col.name)
+		if err != nil {
+			return fmt.Errorf("failed to check if column %s exists: %w", col.name, err)
+		}
+
+		if !exists {
+			if _, err := db.Exec(col.definition); err != nil {
+				return fmt.Errorf("failed to add column %s: %w", col.name, err)
+			}
+		}
+	}
+
+	// Initialize updated_at with created_at for existing records
+	if _, err := db.Exec("UPDATE habits SET updated_at = created_at WHERE updated_at IS NULL"); err != nil {
+		return fmt.Errorf("failed to initialize updated_at: %w", err)
+	}
+
+	// Columns to add to habit_entries table
+	entryColumns := []struct {
+		name       string
+		definition string
+	}{
+		{"updated_at", "ALTER TABLE habit_entries ADD COLUMN updated_at DATETIME"},
+		{"deleted_at", "ALTER TABLE habit_entries ADD COLUMN deleted_at DATETIME"},
+	}
+
+	for _, col := range entryColumns {
+		exists, err := columnExists(db, "habit_entries", col.name)
+		if err != nil {
+			return fmt.Errorf("failed to check if column %s exists: %w", col.name, err)
+		}
+
+		if !exists {
+			if _, err := db.Exec(col.definition); err != nil {
+				return fmt.Errorf("failed to add column %s: %w", col.name, err)
+			}
+		}
+	}
+
+	// Initialize updated_at with completed_at for existing entries
+	if _, err := db.Exec("UPDATE habit_entries SET updated_at = completed_at WHERE updated_at IS NULL"); err != nil {
+		return fmt.Errorf("failed to initialize updated_at for entries: %w", err)
+	}
+
+	// Create indexes for sync queries
+	indexes := []struct {
+		name       string
+		definition string
+	}{
+		{"idx_habits_updated_at", "CREATE INDEX IF NOT EXISTS idx_habits_updated_at ON habits(user_id, updated_at)"},
+		{"idx_habits_deleted_at", "CREATE INDEX IF NOT EXISTS idx_habits_deleted_at ON habits(deleted_at)"},
+		{"idx_habit_entries_updated_at", "CREATE INDEX IF NOT EXISTS idx_habit_entries_updated_at ON habit_entries(habit_id, updated_at)"},
+		{"idx_habit_entries_deleted_at", "CREATE INDEX IF NOT EXISTS idx_habit_entries_deleted_at ON habit_entries(deleted_at)"},
+	}
+
+	for _, idx := range indexes {
+		exists, err := indexExists(db, idx.name)
+		if err != nil {
+			return fmt.Errorf("failed to check if index %s exists: %w", idx.name, err)
+		}
+
+		if !exists {
+			if _, err := db.Exec(idx.definition); err != nil {
+				return fmt.Errorf("failed to create index %s: %w", idx.name, err)
+			}
+		}
+	}
+
+	return nil
+}
+
 const createUsersTable = `
 CREATE TABLE IF NOT EXISTS users (
 	id TEXT PRIMARY KEY,