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/sync/sync-secret.guard.ts

@@ -0,0 +1,18 @@
+import { CanActivate, ExecutionContext, ForbiddenException, Injectable } from '@nestjs/common';
+import { ConfigService } from '@nestjs/config';
+import { Request } from 'express';
+
+/// Server-to-server auth for /sync/*: a shared secret header, not a user token.
+@Injectable()
+export class SyncSecretGuard implements CanActivate {
+  constructor(private readonly config: ConfigService) {}
+
+  canActivate(context: ExecutionContext): boolean {
+    const request = context.switchToHttp().getRequest<Request>();
+    const expected = this.config.getOrThrow<string>('SYNC_SHARED_SECRET');
+    if (request.headers['x-sync-secret'] !== expected) {
+      throw new ForbiddenException('Invalid sync secret');
+    }
+    return true;
+  }
+}