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

frontend/app/[locale]/fahrzeuge/page.tsx

@@ -0,0 +1,5 @@
+import { VehicleList } from '@/components/vehicles/VehicleList';
+
+export default function FahrzeugePage() {
+  return <VehicleList />;
+}