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/auth/authenticated-request.ts

@@ -1,5 +1,6 @@
 import { Request } from 'express';
 import { Role } from '../common/role.enum';
+import { GuestJwtPayload } from './guest-auth.service';
 
 export interface AuthenticatedMembership {
   kcId: string;
@@ -18,3 +19,8 @@ export interface AuthenticatedUser {
 export interface AuthenticatedRequest extends Request {
   user?: AuthenticatedUser;
 }
+
+/// Shape attached to req.user by GuestJwtStrategy for guest/Konfi-authenticated routes.
+export interface GuestAuthenticatedRequest extends Request {
+  user?: GuestJwtPayload;
+}