feat(backend): add GemeindeController for LT congregation CRUD

Fills the plan's known gap where Gemeinde existed only as a Prisma model.
GemeindeModule exposes Leitungsteam-only create/list/get/update/delete
under /api/gemeinde, each mutation captured into the sync log like the
other feature services. Unique-name-per-KC violations surface as 409.
Docs (plan + backend README) updated to drop the gap and next-step item.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/README.md

@@ -36,6 +36,10 @@ client's host - no separate web server is needed.
 - `auth/` — Authentik resource-server strategy (`AuthGuard('authentik')`) +
   guest invite-code login issuing a locally-signed JWT (`AuthGuard('guest')`).
 - `kc/` — KC (event) creation/listing, Leitungsteam-only.
+- `gemeinde/` — Gemeinde (congregation) CRUD per KC (`POST /gemeinde`,
+  `GET /gemeinde?kcId=`, `GET/PATCH/DELETE /gemeinde/:id`), Leitungsteam-only.
+  Gemeinde Verantwortliche/Teamer get their own Gemeinde from their
+  `Membership`, not from this endpoint.
 - `wahl/` — Wahl/Workshop administration (Leitungsteam-only), guest
   Teilnehmer submission, Force-Zuteilung overrides, and `ZuteilungService`:
   a faithful port of the WP plugin's `kc_run_zuteilung` (force-assignments →