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>

src/gemeinde/gemeinde.module.ts

@@ -0,0 +1,9 @@
+import { Module } from '@nestjs/common';
+import { GemeindeService } from './gemeinde.service';
+import { GemeindeController } from './gemeinde.controller';
+
+@Module({
+  providers: [GemeindeService],
+  controllers: [GemeindeController],
+})
+export class GemeindeModule {}