feat(T02): vehicle management + mobile.de push + vehicle UI

- Vehicle model: 25+ fields, 5 vehicle types, soft-delete
- Vehicle CRUD: 7 API endpoints with JWT auth, filter/sort/paginate
- mobile.de: push/update/delete listings, field mapping, retry logic
- MobileDeListing model for sync status tracking
- Frontend: VehicleList, VehicleForm, VehicleDetail, MobileDeStatus
- 73 backend tests (82% coverage), 16 frontend tests

backend/app/config.py

@@ -31,6 +31,8 @@ class Settings(BaseSettings):
     UPLOAD_DIR: str = "/tmp/uploads"
     APP_NAME: str = "ERP Nutzfahrzeuge"
     APP_ENV: str = "development"
+    MOBILE_DE_API_KEY: str = ""
+    MOBILE_DE_SELLER_ID: str = ""
 
     @property
     def cors_origins_list(self) -> list[str]: