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>

internal/services/sync/service.go

@@ -1,24 +1,28 @@
 package sync
 
 import (
+	"encoding/json"
 	"fmt"
 	"log/slog"
 	"time"
 
 	"git.mchus.pro/mchus/quoteforge/internal/localdb"
+	"git.mchus.pro/mchus/quoteforge/internal/models"
 	"git.mchus.pro/mchus/quoteforge/internal/repository"
 )
 
 // Service handles synchronization between MariaDB and local SQLite
 type Service struct {
 	pricelistRepo *repository.PricelistRepository
+	configRepo    *repository.ConfigurationRepository
 	localDB       *localdb.LocalDB
 }
 
 // NewService creates a new sync service
-func NewService(pricelistRepo *repository.PricelistRepository, localDB *localdb.LocalDB) *Service {
+func NewService(pricelistRepo *repository.PricelistRepository, configRepo *repository.ConfigurationRepository, localDB *localdb.LocalDB) *Service {
 	return &Service{
 		pricelistRepo: pricelistRepo,
+		configRepo:    configRepo,
 		localDB:       localDB,
 	}
 }
@@ -213,3 +217,157 @@ func (s *Service) GetPricelistForOffline(serverPricelistID uint) (*localdb.Local
 
 	return localPL, nil
 }
+
+// SyncPricelistsIfNeeded checks for new pricelists and syncs if needed
+// This should be called before creating a new configuration when online
+func (s *Service) SyncPricelistsIfNeeded() error {
+	needSync, err := s.NeedSync()
+	if err != nil {
+		slog.Warn("failed to check if sync needed", "error", err)
+		return nil // Don't fail on check error
+	}
+
+	if !needSync {
+		slog.Debug("pricelists are up to date, no sync needed")
+		return nil
+	}
+
+	slog.Info("new pricelists detected, syncing...")
+	_, err = s.SyncPricelists()
+	if err != nil {
+		return fmt.Errorf("syncing pricelists: %w", err)
+	}
+
+	return nil
+}
+
+// PushPendingChanges pushes all pending changes to the server
+func (s *Service) PushPendingChanges() (int, error) {
+	changes, err := s.localDB.GetPendingChanges()
+	if err != nil {
+		return 0, fmt.Errorf("getting pending changes: %w", err)
+	}
+
+	if len(changes) == 0 {
+		slog.Debug("no pending changes to push")
+		return 0, nil
+	}
+
+	slog.Info("pushing pending changes", "count", len(changes))
+	pushed := 0
+	var syncedIDs []int64
+
+	for _, change := range changes {
+		err := s.pushSingleChange(&change)
+		if err != nil {
+			slog.Warn("failed to push change", "id", change.ID, "type", change.EntityType, "operation", change.Operation, "error", err)
+			// Increment attempts
+			s.localDB.IncrementPendingChangeAttempts(change.ID, err.Error())
+			continue
+		}
+
+		syncedIDs = append(syncedIDs, change.ID)
+		pushed++
+	}
+
+	// Mark synced changes as complete by deleting them
+	if len(syncedIDs) > 0 {
+		if err := s.localDB.MarkChangesSynced(syncedIDs); err != nil {
+			slog.Error("failed to mark changes as synced", "error", err)
+		}
+	}
+
+	slog.Info("pending changes pushed", "pushed", pushed, "failed", len(changes)-pushed)
+	return pushed, nil
+}
+
+// pushSingleChange pushes a single pending change to the server
+func (s *Service) pushSingleChange(change *localdb.PendingChange) error {
+	switch change.EntityType {
+	case "configuration":
+		return s.pushConfigurationChange(change)
+	default:
+		return fmt.Errorf("unknown entity type: %s", change.EntityType)
+	}
+}
+
+// pushConfigurationChange pushes a configuration change to the server
+func (s *Service) pushConfigurationChange(change *localdb.PendingChange) error {
+	switch change.Operation {
+	case "create":
+		return s.pushConfigurationCreate(change)
+	case "update":
+		return s.pushConfigurationUpdate(change)
+	case "delete":
+		return s.pushConfigurationDelete(change)
+	default:
+		return fmt.Errorf("unknown operation: %s", change.Operation)
+	}
+}
+
+// pushConfigurationCreate creates a configuration on the server
+func (s *Service) pushConfigurationCreate(change *localdb.PendingChange) error {
+	var cfg models.Configuration
+	if err := json.Unmarshal([]byte(change.Payload), &cfg); err != nil {
+		return fmt.Errorf("unmarshaling configuration: %w", err)
+	}
+
+	// Create on server
+	if err := s.configRepo.Create(&cfg); err != nil {
+		return fmt.Errorf("creating configuration on server: %w", err)
+	}
+
+	// Update local configuration with server ID
+	localCfg, err := s.localDB.GetConfigurationByUUID(cfg.UUID)
+	if err == nil {
+		serverID := cfg.ID
+		localCfg.ServerID = &serverID
+		localCfg.SyncStatus = "synced"
+		s.localDB.SaveConfiguration(localCfg)
+	}
+
+	slog.Info("configuration created on server", "uuid", cfg.UUID, "server_id", cfg.ID)
+	return nil
+}
+
+// pushConfigurationUpdate updates a configuration on the server
+func (s *Service) pushConfigurationUpdate(change *localdb.PendingChange) error {
+	var cfg models.Configuration
+	if err := json.Unmarshal([]byte(change.Payload), &cfg); err != nil {
+		return fmt.Errorf("unmarshaling configuration: %w", err)
+	}
+
+	// Update on server
+	if err := s.configRepo.Update(&cfg); err != nil {
+		return fmt.Errorf("updating configuration on server: %w", err)
+	}
+
+	// Update local sync status
+	localCfg, err := s.localDB.GetConfigurationByUUID(cfg.UUID)
+	if err == nil {
+		localCfg.SyncStatus = "synced"
+		s.localDB.SaveConfiguration(localCfg)
+	}
+
+	slog.Info("configuration updated on server", "uuid", cfg.UUID)
+	return nil
+}
+
+// pushConfigurationDelete deletes a configuration from the server
+func (s *Service) pushConfigurationDelete(change *localdb.PendingChange) error {
+	// Get the configuration from server by UUID to get the ID
+	cfg, err := s.configRepo.GetByUUID(change.EntityUUID)
+	if err != nil {
+		// Already deleted or not found, consider it successful
+		slog.Warn("configuration not found on server, considering delete successful", "uuid", change.EntityUUID)
+		return nil
+	}
+
+	// Delete from server
+	if err := s.configRepo.Delete(cfg.ID); err != nil {
+		return fmt.Errorf("deleting configuration from server: %w", err)
+	}
+
+	slog.Info("configuration deleted from server", "uuid", change.EntityUUID)
+	return nil
+}