feat(backend): implement phases 0-6 (auth, kc, wahl, files, chat, sync)

Full NestJS backend for the KC-App platform:
- auth: Authentik OIDC resource-server strategy + guest invite-code JWT
  login, plus TokenVerificationService for the WS handshake path
- kc: Leitungsteam-only KC (event) creation/listing
- wahl: Wahl/Workshop admin, Force-Zuteilung overrides, ZuteilungService
  (port of the WP plugin's kc_run_zuteilung), CSV export
- files: LT-only upload with visibility tiers; list/download filtered by
  caller tier; StorageProvider abstraction (WebDAV/Nextcloud default, S3)
- chat: Gemeinde group / DM / LT-wide / broadcast channels; REST + raw ws
  gateway sharing ChatService access rules
- sync: append-only SyncLogEntry replication log + local<->cloud
  push/pull scheduler, shared-secret guarded
- common: Role enum, @Roles decorator, KC-scoped RolesGuard (LT global)
- serves client/web/ interim static web client under / (API under /api)

Typecheck, nest build and boot test pass; needs real Postgres/Authentik/
Nextcloud to run end to end.

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

src/chat/dto/create-channel.dto.ts

@@ -0,0 +1,11 @@
+import { IsEnum, IsOptional, IsString } from 'class-validator';
+import { ChatChannelType } from '@prisma/client';
+
+export class CreateChannelDto {
+  @IsEnum(ChatChannelType)
+  type!: ChatChannelType;
+
+  @IsOptional()
+  @IsString()
+  gemeindeId?: string;
+}