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>

backend/src/auth/auth.module.ts

@@ -5,6 +5,8 @@ import { PassportModule } from '@nestjs/passport';
 import { AuthController } from './auth.controller';
 import { GuestAuthService } from './guest-auth.service';
 import { AuthentikStrategy } from './authentik.strategy';
+import { GuestJwtStrategy } from './guest-jwt.strategy';
+import { TokenVerificationService } from './token-verification.service';
 
 @Module({
   imports: [
@@ -18,6 +20,7 @@ import { AuthentikStrategy } from './authentik.strategy';
     }),
   ],
   controllers: [AuthController],
-  providers: [GuestAuthService, AuthentikStrategy],
+  providers: [GuestAuthService, AuthentikStrategy, GuestJwtStrategy, TokenVerificationService],
+  exports: [TokenVerificationService],
 })
 export class AuthModule {}