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/localdb/localdb.go

@@ -56,6 +56,7 @@ func New(dbPath string) (*LocalDB, error) {
 		&LocalPricelistItem{},
 		&LocalComponent{},
 		&AppSetting{},
+		&PendingChange{},
 	); err != nil {
 		return nil, fmt.Errorf("migrating sqlite database: %w", err)
 	}
@@ -337,3 +338,73 @@ func (l *LocalDB) DeleteLocalPricelist(id uint) error {
 	// Delete pricelist
 	return l.db.Delete(&LocalPricelist{}, id).Error
 }
+
+// PendingChange methods
+
+// AddPendingChange adds a change to the sync queue
+func (l *LocalDB) AddPendingChange(entityType, entityUUID, operation, payload string) error {
+	change := PendingChange{
+		EntityType: entityType,
+		EntityUUID: entityUUID,
+		Operation:  operation,
+		Payload:    payload,
+		CreatedAt:  time.Now(),
+		Attempts:   0,
+	}
+	return l.db.Create(&change).Error
+}
+
+// GetPendingChanges returns all pending changes ordered by creation time
+func (l *LocalDB) GetPendingChanges() ([]PendingChange, error) {
+	var changes []PendingChange
+	err := l.db.Order("created_at ASC").Find(&changes).Error
+	return changes, err
+}
+
+// GetPendingChangesByEntity returns pending changes for a specific entity
+func (l *LocalDB) GetPendingChangesByEntity(entityType, entityUUID string) ([]PendingChange, error) {
+	var changes []PendingChange
+	err := l.db.Where("entity_type = ? AND entity_uuid = ?", entityType, entityUUID).
+		Order("created_at ASC").Find(&changes).Error
+	return changes, err
+}
+
+// DeletePendingChange removes a change from the sync queue after successful sync
+func (l *LocalDB) DeletePendingChange(id int64) error {
+	return l.db.Delete(&PendingChange{}, id).Error
+}
+
+// IncrementPendingChangeAttempts updates the attempt counter and last error
+func (l *LocalDB) IncrementPendingChangeAttempts(id int64, errorMsg string) error {
+	return l.db.Model(&PendingChange{}).Where("id = ?", id).Updates(map[string]interface{}{
+		"attempts":   gorm.Expr("attempts + 1"),
+		"last_error": errorMsg,
+	}).Error
+}
+
+// CountPendingChanges returns the total number of pending changes
+func (l *LocalDB) CountPendingChanges() int64 {
+	var count int64
+	l.db.Model(&PendingChange{}).Count(&count)
+	return count
+}
+
+// CountPendingChangesByType returns the number of pending changes by entity type
+func (l *LocalDB) CountPendingChangesByType(entityType string) int64 {
+	var count int64
+	l.db.Model(&PendingChange{}).Where("entity_type = ?", entityType).Count(&count)
+	return count
+}
+
+// MarkChangesSynced marks multiple pending changes as synced by deleting them
+func (l *LocalDB) MarkChangesSynced(ids []int64) error {
+	if len(ids) == 0 {
+		return nil
+	}
+	return l.db.Where("id IN ?", ids).Delete(&PendingChange{}).Error
+}
+
+// GetPendingCount returns the total number of pending changes (alias for CountPendingChanges)
+func (l *LocalDB) GetPendingCount() int64 {
+	return l.CountPendingChanges()
+}