feat(T04): contact management + USt-IdNr. validation + contact UI

- Contact model: company, role (kaeufer/verkaeufer/beide), soft-delete
- ContactPerson model: 1:N relation with CASCADE delete
- USt-IdNr. validation: DE + 10 EU countries
- Contact CRUD: 7 API endpoints with RBAC, search/filter/paginate
- Frontend: ContactList, ContactForm, ContactDetail with EU/Inland toggle
- 73 backend tests (91% coverage), 20 frontend tests

backend/app/main.py

@@ -9,7 +9,7 @@ from fastapi import APIRouter, FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
 from app.config import settings
-from app.routers import auth, users, vehicles
+from app.routers import auth, contacts, users, vehicles
 
 
 @asynccontextmanager
@@ -41,6 +41,7 @@ api_v1_router = APIRouter(prefix="/api/v1")
 api_v1_router.include_router(auth.router)
 api_v1_router.include_router(users.router)
 api_v1_router.include_router(vehicles.router)
+api_v1_router.include_router(contacts.router)
 
 # Health endpoint (no auth required)
 @api_v1_router.get("/health", tags=["health"])