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/files/visibility.util.ts

@@ -0,0 +1,21 @@
+import { FileVisibility, Role } from '@prisma/client';
+import { AuthenticatedUser } from '../auth/authenticated-request';
+
+/// Maps the caller's role for a given KC to the file visibility tiers they may see.
+/// LEITUNGSTEAM is global (per RolesGuard convention) and sees everything.
+export function allowedVisibilitiesForUser(
+  user: AuthenticatedUser,
+  kcId: string,
+): FileVisibility[] {
+  const isLt = user.memberships.some((m) => m.role === Role.LEITUNGSTEAM);
+  if (isLt) {
+    return [FileVisibility.ALLE, FileVisibility.ALLE_AUSSER_KONFIS, FileVisibility.NUR_LT];
+  }
+  const isTeamMemberForKc = user.memberships.some((m) => m.kcId === kcId);
+  if (isTeamMemberForKc) {
+    return [FileVisibility.ALLE, FileVisibility.ALLE_AUSSER_KONFIS];
+  }
+  return [];
+}
+
+export const GUEST_ALLOWED_VISIBILITIES: FileVisibility[] = [FileVisibility.ALLE];