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/handlers/export.go

@@ -12,13 +12,13 @@ import (
 
 type ExportHandler struct {
 	exportService    *services.ExportService
-	configService    *services.ConfigurationService
+	configService    services.ConfigurationGetter
 	componentService *services.ComponentService
 }
 
 func NewExportHandler(
 	exportService *services.ExportService,
-	configService *services.ConfigurationService,
+	configService services.ConfigurationGetter,
 	componentService *services.ComponentService,
 ) *ExportHandler {
 	return &ExportHandler{