feat(backend): push notifications module (FCM HTTP v1)

New global push/ module mirroring mail/ and files/storage/:
- PushProvider abstraction; default LogPushProvider (no delivery, logs),
  PUSH_PROVIDER=fcm switches to FcmPushProvider — Firebase Cloud Messaging
  HTTP v1, authenticated by a service-account JWT exchanged for an OAuth
  token (no extra dependency; jsonwebtoken does the signing). Prunes tokens
  FCM reports as invalid.
- DeviceToken model (token + platform, bound to a User or GuestAccount),
  migration + added to the sync log.
- POST /api/push/register + /unregister (any of the three token kinds).
- PushService.notifyChannel() resolves a channel's readable audience
  (DIREKT participants / LT / Gemeinde members + guests / whole KC for
  broadcast), looks up their device tokens (minus the sender), sends.
- ChatService.sendMessage() fires it best-effort after persisting.

New env: PUSH_PROVIDER, FCM_PROJECT_ID (default konfi-castle-app),
GOOGLE_APPLICATION_CREDENTIALS.

Verified against local Postgres: register a token, send a Gemeinde-group
chat message from another member -> log-push logs "would push ... to 1
device". Real FCM send needs the service-account JSON. npm test 56.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app.module.ts

@@ -5,6 +5,7 @@ import { existsSync } from 'fs';
 import { join } from 'path';
 import { PrismaModule } from './prisma/prisma.module';
 import { MailModule } from './mail/mail.module';
+import { PushModule } from './push/push.module';
 import { AuthModule } from './auth/auth.module';
 import { KcModule } from './kc/kc.module';
 import { GemeindeModule } from './gemeinde/gemeinde.module';
@@ -35,6 +36,7 @@ const webRoot =
     }),
     PrismaModule,
     MailModule,
+    PushModule,
     SyncModule,
     AuthModule,
     KcModule,