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/router.go

@@ -17,7 +17,7 @@ import (
 	_ "apocapoc-api/docs"
 )
 
-func NewRouter(appURL string, habitHandlers *HabitHandlers, authHandlers *AuthHandlers, statsHandlers *StatsHandlers, healthHandlers *HealthHandlers, userHandlers *UserHandlers, exportHandlers *ExportHandlers, jwtService *auth.JWTService, translator *i18n.Translator) *chi.Mux {
+func NewRouter(appURL string, habitHandlers *HabitHandlers, authHandlers *AuthHandlers, statsHandlers *StatsHandlers, healthHandlers *HealthHandlers, userHandlers *UserHandlers, exportHandlers *ExportHandlers, syncHandlers *SyncHandlers, jwtService *auth.JWTService, translator *i18n.Translator) *chi.Mux {
 	r := chi.NewRouter()
 
 	r.Use(logger.Middleware)
@@ -86,5 +86,12 @@ func NewRouter(appURL string, habitHandlers *HabitHandlers, authHandlers *AuthHa
 		r.Get("/", exportHandlers.ExportData)
 	})
 
+	r.Route("/api/v1/sync", func(r chi.Router) {
+		r.Use(AuthMiddleware(jwtService))
+		r.Use(RateLimitByUser(jwtService, 100, 1*time.Minute))
+		r.Get("/changes", syncHandlers.GetSyncChanges)
+		r.Post("/batch", syncHandlers.ApplySyncBatch)
+	})
+
 	return r
 }