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/app.module.ts

@@ -1,15 +1,31 @@
 import { Module } from '@nestjs/common';
 import { ConfigModule } from '@nestjs/config';
+import { ServeStaticModule } from '@nestjs/serve-static';
+import { join } from 'path';
 import { PrismaModule } from './prisma/prisma.module';
 import { AuthModule } from './auth/auth.module';
 import { KcModule } from './kc/kc.module';
+import { WahlModule } from './wahl/wahl.module';
+import { FilesModule } from './files/files.module';
+import { ChatModule } from './chat/chat.module';
+import { SyncModule } from './sync/sync.module';
 
 @Module({
   imports: [
     ConfigModule.forRoot({ isGlobal: true }),
+    // Serves the plain static web client from ../client/web; the REST API
+    // lives under /api (see main.ts) so it never collides with these routes.
+    ServeStaticModule.forRoot({
+      rootPath: join(__dirname, '..', '..', 'client', 'web'),
+      exclude: ['/api*'],
+    }),
     PrismaModule,
+    SyncModule,
     AuthModule,
     KcModule,
+    WahlModule,
+    FilesModule,
+    ChatModule,
   ],
 })
 export class AppModule {}