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/chat/chat.controller.ts

@@ -0,0 +1,45 @@
+import { Body, Controller, Get, Param, Post, Req, UseGuards } from '@nestjs/common';
+import { AuthGuard } from '@nestjs/passport';
+import { ChatService } from './chat.service';
+import { CreateChannelDto } from './dto/create-channel.dto';
+import { CreateDirectChannelDto } from './dto/create-direct-channel.dto';
+import { Roles } from '../common/roles.decorator';
+import { RolesGuard } from '../common/roles.guard';
+import { Role } from '../common/role.enum';
+import { AuthenticatedRequest } from '../auth/authenticated-request';
+import { GuestJwtPayload } from '../auth/guest-auth.service';
+import { resolveChatCaller } from './caller.util';
+
+type ChatRequest = AuthenticatedRequest & { user?: AuthenticatedRequest['user'] | GuestJwtPayload };
+
+@Controller('chat')
+export class ChatController {
+  constructor(private readonly chat: ChatService) {}
+
+  /// Channel administration (Gemeinde-Gruppen, LT-Kanäle, Broadcasts) is Leitungsteam-only.
+  @Post(':kcId/channels')
+  @UseGuards(AuthGuard('authentik'), RolesGuard)
+  @Roles(Role.LEITUNGSTEAM)
+  createChannel(@Param('kcId') kcId: string, @Body() dto: CreateChannelDto) {
+    return this.chat.createChannel(kcId, dto.type, dto.gemeindeId);
+  }
+
+  /// Any two team members of the same KC can start a direct conversation.
+  @Post('direct')
+  @UseGuards(AuthGuard('authentik'))
+  createDirectChannel(@Body() dto: CreateDirectChannelDto, @Req() req: AuthenticatedRequest) {
+    return this.chat.getOrCreateDirectChannel(dto.kcId, req.user!.userId, dto.otherUserId);
+  }
+
+  @Get(':kcId/channels')
+  @UseGuards(AuthGuard(['authentik', 'guest']))
+  listChannels(@Param('kcId') kcId: string, @Req() req: ChatRequest) {
+    return this.chat.listChannelsForCaller(kcId, resolveChatCaller(req.user!));
+  }
+
+  @Get('channels/:channelId/messages')
+  @UseGuards(AuthGuard(['authentik', 'guest']))
+  listMessages(@Param('channelId') channelId: string, @Req() req: ChatRequest) {
+    return this.chat.listMessages(channelId, resolveChatCaller(req.user!));
+  }
+}