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/http/integration_test.go

@@ -70,14 +70,18 @@ func setupTestServer(t *testing.T) *TestServer {
 
 	translator, _ := i18n.NewTranslator()
 
+	getSyncChangesHandler := queries.NewGetSyncChangesHandler(habitRepo, entryRepo)
+	applySyncBatchHandler := commands.NewApplySyncBatchHandler(habitRepo, entryRepo)
+
 	authHandlers := NewAuthHandlers(registerHandler, loginHandler, refreshTokenHandler, revokeTokenHandler, revokeAllTokensHandler, verifyEmailHandler, resendVerificationEmailHandler, requestPasswordResetHandler, resetPasswordHandler, jwtService, refreshTokenRepo, refreshTokenExpiry, translator)
 	habitHandlers := NewHabitHandlers(createHandler, getTodaysHandler, getUserHabitsHandler, getHabitByIDHandler, getHabitEntriesHandler, updateHandler, archiveHandler, markHandler, unmarkHandler, translator)
 	statsHandlers := NewStatsHandlers(getHabitStatsHandler, translator)
 	healthHandlers := NewHealthHandlers(db, nil)
 	userHandlers := NewUserHandlers(deleteUserHandler, translator)
 	exportHandlers := NewExportHandlers(exportUserDataHandler, translator)
+	syncHandlers := NewSyncHandlers(getSyncChangesHandler, applySyncBatchHandler, translator)
 
-	router := NewRouter("http://localhost:3000", habitHandlers, authHandlers, statsHandlers, healthHandlers, userHandlers, exportHandlers, jwtService, translator)
+	router := NewRouter("http://localhost:3000", habitHandlers, authHandlers, statsHandlers, healthHandlers, userHandlers, exportHandlers, syncHandlers, jwtService, translator)
 
 	handler := http.Handler(router)
 	return &TestServer{