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/src/gemeinde/dto/create-gemeinde.dto.ts

@@ -0,0 +1,11 @@
+import { IsNotEmpty, IsString } from 'class-validator';
+
+export class CreateGemeindeDto {
+  @IsString()
+  @IsNotEmpty()
+  kcId!: string;
+
+  @IsString()
+  @IsNotEmpty()
+  name!: string;
+}