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/application/queries/get_sync_changes.go

@@ -0,0 +1,76 @@
+package queries
+
+import (
+	"context"
+	"time"
+
+	"apocapoc-api/internal/domain/entities"
+	"apocapoc-api/internal/domain/repositories"
+	"apocapoc-api/internal/shared/errors"
+)
+
+type HabitChangesDTO struct {
+	Created []*entities.Habit
+	Updated []*entities.Habit
+	Deleted []string
+}
+
+type EntryChangesDTO struct {
+	Created []*entities.HabitEntry
+	Updated []*entities.HabitEntry
+	Deleted []string
+}
+
+type SyncChangesDTO struct {
+	Habits  HabitChangesDTO
+	Entries EntryChangesDTO
+}
+
+type GetSyncChangesQuery struct {
+	UserID string
+	Since  time.Time
+}
+
+type GetSyncChangesHandler struct {
+	habitRepo repositories.HabitRepository
+	entryRepo repositories.HabitEntryRepository
+}
+
+func NewGetSyncChangesHandler(
+	habitRepo repositories.HabitRepository,
+	entryRepo repositories.HabitEntryRepository,
+) *GetSyncChangesHandler {
+	return &GetSyncChangesHandler{
+		habitRepo: habitRepo,
+		entryRepo: entryRepo,
+	}
+}
+
+func (h *GetSyncChangesHandler) Handle(ctx context.Context, query GetSyncChangesQuery) (*SyncChangesDTO, error) {
+	if query.UserID == "" {
+		return nil, errors.ErrInvalidInput
+	}
+
+	habitChanges, err := h.habitRepo.GetChangesSince(ctx, query.UserID, query.Since)
+	if err != nil {
+		return nil, err
+	}
+
+	entryChanges, err := h.entryRepo.GetChangesSince(ctx, query.UserID, query.Since)
+	if err != nil {
+		return nil, err
+	}
+
+	return &SyncChangesDTO{
+		Habits: HabitChangesDTO{
+			Created: habitChanges.Created,
+			Updated: habitChanges.Updated,
+			Deleted: habitChanges.Deleted,
+		},
+		Entries: EntryChangesDTO{
+			Created: entryChanges.Created,
+			Updated: entryChanges.Updated,
+			Deleted: entryChanges.Deleted,
+		},
+	}, nil
+}