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/app.module.ts

@@ -5,6 +5,7 @@ import { join } from 'path';
 import { PrismaModule } from './prisma/prisma.module';
 import { AuthModule } from './auth/auth.module';
 import { KcModule } from './kc/kc.module';
+import { GemeindeModule } from './gemeinde/gemeinde.module';
 import { WahlModule } from './wahl/wahl.module';
 import { FilesModule } from './files/files.module';
 import { ChatModule } from './chat/chat.module';
@@ -23,6 +24,7 @@ import { SyncModule } from './sync/sync.module';
     SyncModule,
     AuthModule,
     KcModule,
+    GemeindeModule,
     WahlModule,
     FilesModule,
     ChatModule,