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/configuration.go

@@ -14,6 +14,12 @@ var (
 	ErrConfigForbidden  = errors.New("access to configuration forbidden")
 )
 
+// ConfigurationGetter is an interface for services that can retrieve configurations
+// Used by handlers to work with both ConfigurationService and LocalConfigurationService
+type ConfigurationGetter interface {
+	GetByUUID(uuid string, userID uint) (*models.Configuration, error)
+}
+
 type ConfigurationService struct {
 	configRepo    *repository.ConfigurationRepository
 	componentRepo *repository.ComponentRepository