Add car_id field to RequestVehicleResponse schema

This field was missing, causing the API to not return car_id
for approved vehicles in My Vehicle Requests page.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/schemas/vehicle_request.py

@@ -56,6 +56,7 @@ class RequestVehicleCreate(BaseModel):
 class RequestVehicleResponse(BaseModel):
     id: int
     request_id: int
+    car_id: Optional[int] = None  # Reference to cars table
     car_data: dict
     is_approved: bool
     approved_at: Optional[datetime]