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 ✓

cmd/api/main.go

@@ -141,6 +141,8 @@ func main() {
 	archiveHandler := commands.NewArchiveHabitHandler(habitRepo)
 	markHandler := commands.NewMarkHabitHandler(entryRepo, habitRepo)
 	unmarkHandler := commands.NewUnmarkHabitHandler(habitRepo, entryRepo)
+	getSyncChangesHandler := queries.NewGetSyncChangesHandler(habitRepo, entryRepo)
+	applySyncBatchHandler := commands.NewApplySyncBatchHandler(habitRepo, entryRepo)
 
 	authHandlers := httpInfra.NewAuthHandlers(registerHandler, loginHandler, refreshTokenHandler, revokeTokenHandler, revokeAllTokensHandler, verifyEmailHandler, resendVerificationEmailHandler, requestPasswordResetHandler, resetPasswordHandler, jwtService, refreshTokenRepo, refreshTokenExpiry, translator)
 	habitHandlers := httpInfra.NewHabitHandlers(createHandler, getTodaysHandler, getUserHabitsHandler, getHabitByIDHandler, getHabitEntriesHandler, updateHandler, archiveHandler, markHandler, unmarkHandler, translator)
@@ -148,8 +150,9 @@ func main() {
 	healthHandlers := httpInfra.NewHealthHandlers(db.Conn(), emailService)
 	userHandlers := httpInfra.NewUserHandlers(deleteUserHandler, translator)
 	exportHandlers := httpInfra.NewExportHandlers(exportUserDataHandler, translator)
+	syncHandlers := httpInfra.NewSyncHandlers(getSyncChangesHandler, applySyncBatchHandler, translator)
 
-	router := httpInfra.NewRouter(cfg.AppURL, habitHandlers, authHandlers, statsHandlers, healthHandlers, userHandlers, exportHandlers, jwtService, translator)
+	router := httpInfra.NewRouter(cfg.AppURL, habitHandlers, authHandlers, statsHandlers, healthHandlers, userHandlers, exportHandlers, syncHandlers, jwtService, translator)
 
 	addr := fmt.Sprintf("0.0.0.0:%s", cfg.Port)
 	logger.Info().Str("address", addr).Msg("Server starting")