Add sync conflict detection and resolution system

Implement conflict detection for concurrent reading progress updates from different devices. Adds conflict management endpoints for listing, viewing, and resolving conflicts.

- Add ConflictHandler with CRUD endpoints for conflict management
- Implement automatic conflict detection in KOReader progress updates
- Add WebSocket broadcast for real-time conflict notifications
- Add database query for listing user conflicts by status
- Add integration tests and Bruno API test collection

internal/sync/websocket.go

@@ -121,6 +121,20 @@ func (m *ConnectionManager) BroadcastAnnotationUpdate(bookID uuid.UUID, annotati
 	m.Broadcast(msg)
 }
 
+// BroadcastConflictNotification broadcasts a conflict notification to all connected clients
+func (m *ConnectionManager) BroadcastConflictNotification(bookID [16]byte, notificationType string, conflictID string) {
+	msg := BroadcastMessage{
+		Type:      MessageTypeConflict,
+		Timestamp: time.Now().Format(time.RFC3339),
+		Data: map[string]interface{}{
+			"book_id":           uuid.UUID(bookID).String(),
+			"notification_type": notificationType,
+			"conflict_id":       conflictID,
+		},
+	}
+	m.Broadcast(msg)
+}
+
 // AddConnection adds a new WebSocket connection
 func (m *ConnectionManager) AddConnection(conn *DeviceConnection) {
 	m.mu.Lock()