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/models.go

@@ -120,3 +120,19 @@ type LocalComponent struct {
 func (LocalComponent) TableName() string {
 	return "local_components"
 }
+
+// PendingChange stores changes that need to be synced to the server
+type PendingChange struct {
+	ID         int64     `gorm:"primaryKey;autoIncrement" json:"id"`
+	EntityType string    `gorm:"not null;index" json:"entity_type"` // "configuration", "project", "specification"
+	EntityUUID string    `gorm:"not null;index" json:"entity_uuid"`
+	Operation  string    `gorm:"not null" json:"operation"` // "create", "update", "delete"
+	Payload    string    `gorm:"type:text" json:"payload"`  // JSON snapshot of the entity
+	CreatedAt  time.Time `gorm:"not null" json:"created_at"`
+	Attempts   int       `gorm:"default:0" json:"attempts"` // Retry count for sync
+	LastError  string    `gorm:"type:text" json:"last_error,omitempty"`
+}
+
+func (PendingChange) TableName() string {
+	return "pending_changes"
+}