fix: Prevent duplicate car creation when adding from All Cars tab

- Backend: Check local_car_id first before creating new car
- Frontend: Pass local_car_id to indicate car already exists in DB

frontend/src/app/admin/cars/page.tsx

@@ -1121,6 +1121,7 @@ export default function CarsAdminPage() {
             request_id: parseInt(requestId),
             car_data: {
               id: car.id.toString(),
+              local_car_id: car.id,  // 로컬 DB에 이미 있는 차량임을 표시
               car_name: car.car_name,
               maker_name: car.maker?.name,
               model_name: car.model?.name,