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

frontend/app/[locale]/kontakte/[id]/page.tsx

@@ -0,0 +1,10 @@
+import { ContactDetail } from '@/components/contacts/ContactDetail';
+
+export default async function KontaktDetailPage({
+  params,
+}: {
+  params: { locale: string; id: string };
+}) {
+  const { id } = params;
+  return <ContactDetail contactId={id} />;
+}