Add background sync worker and complete local-first architecture

Implements automatic background synchronization every 5 minutes:
- Worker pushes pending changes to server (PushPendingChanges)
- Worker pulls new pricelists (SyncPricelistsIfNeeded)
- Graceful shutdown with context cancellation
- Automatic online/offline detection via DB ping

New files:
- internal/services/sync/worker.go - Background sync worker
- internal/services/local_configuration.go - Local-first CRUD
- internal/localdb/converters.go - MariaDB ↔ SQLite converters

Extended sync infrastructure:
- Pending changes queue (pending_changes table)
- Push/pull sync endpoints (/api/sync/push, /pending)
- ConfigurationGetter interface for handler compatibility
- LocalConfigurationService replaces ConfigurationService

All configuration operations now run through SQLite with automatic
background sync to MariaDB when online. Phase 2.5 nearly complete.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

cmd/server/main.go

@@ -108,12 +108,19 @@ func main() {
 	}
 
 	gin.SetMode(cfg.Server.Mode)
-	router, err := setupRouter(db, cfg, local, dbUserID)
+	router, syncService, err := setupRouter(db, cfg, local, dbUserID)
 	if err != nil {
 		slog.Error("failed to setup router", "error", err)
 		os.Exit(1)
 	}
 
+	// Start background sync worker
+	workerCtx, workerCancel := context.WithCancel(context.Background())
+	defer workerCancel()
+
+	syncWorker := sync.NewWorker(syncService, db, 5*time.Minute)
+	go syncWorker.Start(workerCtx)
+
 	srv := &http.Server{
 		Addr:         cfg.Address(),
 		Handler:      router,
@@ -135,6 +142,11 @@ func main() {
 
 	slog.Info("shutting down server...")
 
+	// Stop background sync worker first
+	syncWorker.Stop()
+	workerCancel()
+
+	// Then shutdown HTTP server
 	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 	defer cancel()
 
@@ -282,7 +294,7 @@ func setupDatabaseFromDSN(dsn string) (*gorm.DB, error) {
 	return db, nil
 }
 
-func setupRouter(db *gorm.DB, cfg *config.Config, local *localdb.LocalDB, dbUserID uint) (*gin.Engine, error) {
+func setupRouter(db *gorm.DB, cfg *config.Config, local *localdb.LocalDB, dbUserID uint) (*gin.Engine, *sync.Service, error) {
 	// Repositories
 	componentRepo := repository.NewComponentRepository(db)
 	categoryRepo := repository.NewCategoryRepository(db)
@@ -299,8 +311,19 @@ func setupRouter(db *gorm.DB, cfg *config.Config, local *localdb.LocalDB, dbUser
 	exportService := services.NewExportService(cfg.Export, categoryRepo)
 	alertService := alerts.NewService(alertRepo, componentRepo, priceRepo, statsRepo, cfg.Alerts, cfg.Pricing)
 	pricelistService := pricelist.NewService(db, pricelistRepo, componentRepo)
-	configService := services.NewConfigurationService(configRepo, componentRepo, quoteService)
-	syncService := sync.NewService(pricelistRepo, local)
+	syncService := sync.NewService(pricelistRepo, configRepo, local)
+
+	// isOnline function for local-first architecture
+	isOnline := func() bool {
+		sqlDB, err := db.DB()
+		if err != nil {
+			return false
+		}
+		return sqlDB.Ping() == nil
+	}
+
+	// Local-first configuration service (replaces old ConfigurationService)
+	configService := services.NewLocalConfigurationService(local, syncService, quoteService, isOnline)
 
 	// Handlers
 	componentHandler := handlers.NewComponentHandler(componentService)
@@ -313,13 +336,13 @@ func setupRouter(db *gorm.DB, cfg *config.Config, local *localdb.LocalDB, dbUser
 	// Setup handler (for reconfiguration)
 	setupHandler, err := handlers.NewSetupHandler(local, "web/templates")
 	if err != nil {
-		return nil, fmt.Errorf("creating setup handler: %w", err)
+		return nil, nil, fmt.Errorf("creating setup handler: %w", err)
 	}
 
 	// Web handler (templates)
 	webHandler, err := handlers.NewWebHandler("web/templates", componentService)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	// Router
@@ -584,10 +607,13 @@ func setupRouter(db *gorm.DB, cfg *config.Config, local *localdb.LocalDB, dbUser
 			syncAPI.POST("/components", syncHandler.SyncComponents)
 			syncAPI.POST("/pricelists", syncHandler.SyncPricelists)
 			syncAPI.POST("/all", syncHandler.SyncAll)
+			syncAPI.POST("/push", syncHandler.PushPendingChanges)
+			syncAPI.GET("/pending/count", syncHandler.GetPendingCount)
+			syncAPI.GET("/pending", syncHandler.GetPendingChanges)
 		}
 	}
 
-	return router, nil
+	return router, syncService, nil
 }
 
 func requestLogger() gin.HandlerFunc {