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/domain/entities/habit.go

@@ -19,7 +19,9 @@ type Habit struct {
 	IsNegative    bool
 	TargetValue   *float64
 	CreatedAt     time.Time
+	UpdatedAt     time.Time
 	ArchivedAt    *time.Time
+	DeletedAt     *time.Time
 }
 
 func NewHabit(
@@ -30,6 +32,7 @@ func NewHabit(
 	carryOver bool,
 	isNegative bool,
 ) *Habit {
+	now := time.Now()
 	return &Habit{
 		UserID:     userID,
 		Name:       name,
@@ -37,15 +40,31 @@ func NewHabit(
 		Frequency:  frequency,
 		CarryOver:  carryOver,
 		IsNegative: isNegative,
-		CreatedAt:  time.Now(),
+		CreatedAt:  now,
+		UpdatedAt:  now,
 	}
 }
 
 func (h *Habit) Archive() {
 	now := time.Now()
 	h.ArchivedAt = &now
+	h.UpdatedAt = now
 }
 
 func (h *Habit) IsActive() bool {
 	return h.ArchivedAt == nil
 }
+
+func (h *Habit) Delete() {
+	now := time.Now()
+	h.DeletedAt = &now
+	h.UpdatedAt = now
+}
+
+func (h *Habit) IsDeleted() bool {
+	return h.DeletedAt != nil
+}
+
+func (h *Habit) Touch() {
+	h.UpdatedAt = time.Now()
+}

internal/domain/entities/habit_entry.go

@@ -8,13 +8,27 @@ type HabitEntry struct {
 	ScheduledDate time.Time
 	CompletedAt   time.Time
 	Value         *float64
+	UpdatedAt     time.Time
+	DeletedAt     *time.Time
 }
 
 func NewHabitEntry(habitID string, scheduledDate time.Time, value *float64) *HabitEntry {
+	now := time.Now()
 	return &HabitEntry{
 		HabitID:       habitID,
 		ScheduledDate: scheduledDate,
-		CompletedAt:   time.Now(),
+		CompletedAt:   now,
 		Value:         value,
+		UpdatedAt:     now,
 	}
 }
+
+func (e *HabitEntry) Delete() {
+	now := time.Now()
+	e.DeletedAt = &now
+	e.UpdatedAt = now
+}
+
+func (e *HabitEntry) IsDeleted() bool {
+	return e.DeletedAt != nil
+}

internal/domain/repositories/habit_entry_repository.go

@@ -7,6 +7,12 @@ import (
 	"apocapoc-api/internal/domain/entities"
 )
 
+type HabitEntryChanges struct {
+	Created []*entities.HabitEntry
+	Updated []*entities.HabitEntry
+	Deleted []string
+}
+
 type HabitEntryRepository interface {
 	Create(ctx context.Context, entry *entities.HabitEntry) error
 	FindByID(ctx context.Context, id string) (*entities.HabitEntry, error)
@@ -16,4 +22,8 @@ type HabitEntryRepository interface {
 	FindPendingByHabitID(ctx context.Context, habitID string, beforeDate time.Time) ([]*entities.HabitEntry, error)
 	Update(ctx context.Context, entry *entities.HabitEntry) error
 	Delete(ctx context.Context, id string) error
+
+	// Sync methods
+	GetChangesSince(ctx context.Context, userID string, since time.Time) (*HabitEntryChanges, error)
+	SoftDelete(ctx context.Context, id string) error
 }

internal/domain/repositories/habit_repository.go

@@ -2,6 +2,7 @@ package repositories
 
 import (
 	"context"
+	"time"
 
 	"apocapoc-api/internal/domain/entities"
 	"apocapoc-api/internal/domain/value_objects"
@@ -15,6 +16,12 @@ type HabitFilter struct {
 	Search          string
 }
 
+type HabitChanges struct {
+	Created []*entities.Habit
+	Updated []*entities.Habit
+	Deleted []string
+}
+
 type HabitRepository interface {
 	Create(ctx context.Context, habit *entities.Habit) error
 	FindByID(ctx context.Context, id string) (*entities.Habit, error)
@@ -26,4 +33,8 @@ type HabitRepository interface {
 	CountByUserIDFiltered(ctx context.Context, userID string, filter HabitFilter) (int, error)
 	Update(ctx context.Context, habit *entities.Habit) error
 	Delete(ctx context.Context, id string) error
+
+	// Sync methods
+	GetChangesSince(ctx context.Context, userID string, since time.Time) (*HabitChanges, error)
+	SoftDelete(ctx context.Context, id string) error
 }