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{

internal/handlers/sync.go

@@ -215,3 +215,58 @@ func (h *SyncHandler) checkOnline() bool {
 
 	return true
 }
+
+// PushPendingChanges pushes all pending changes to the server
+// POST /api/sync/push
+func (h *SyncHandler) PushPendingChanges(c *gin.Context) {
+	if !h.checkOnline() {
+		c.JSON(http.StatusServiceUnavailable, gin.H{
+			"success": false,
+			"error":   "Database is offline",
+		})
+		return
+	}
+
+	startTime := time.Now()
+	pushed, err := h.syncService.PushPendingChanges()
+	if err != nil {
+		slog.Error("push pending changes failed", "error", err)
+		c.JSON(http.StatusInternalServerError, gin.H{
+			"success": false,
+			"error":   err.Error(),
+		})
+		return
+	}
+
+	c.JSON(http.StatusOK, SyncResultResponse{
+		Success:  true,
+		Message:  "Pending changes pushed successfully",
+		Synced:   pushed,
+		Duration: time.Since(startTime).String(),
+	})
+}
+
+// GetPendingCount returns the number of pending changes
+// GET /api/sync/pending/count
+func (h *SyncHandler) GetPendingCount(c *gin.Context) {
+	count := h.localDB.GetPendingCount()
+	c.JSON(http.StatusOK, gin.H{
+		"count": count,
+	})
+}
+
+// GetPendingChanges returns all pending changes
+// GET /api/sync/pending
+func (h *SyncHandler) GetPendingChanges(c *gin.Context) {
+	changes, err := h.localDB.GetPendingChanges()
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{
+			"error": err.Error(),
+		})
+		return
+	}
+
+	c.JSON(http.StatusOK, gin.H{
+		"changes": changes,
+	})
+}