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>

backend/src/push/log-push.provider.ts

@@ -0,0 +1,15 @@
+import { Logger } from '@nestjs/common';
+import { PushNotification, PushProvider, PushResult } from './push-provider';
+
+/// Default provider: doesn't send, just logs. Keeps the app working before
+/// FCM credentials are configured.
+export class LogPushProvider implements PushProvider {
+  private readonly logger = new Logger('PushProvider');
+
+  async sendToTokens(tokens: string[], n: PushNotification): Promise<PushResult> {
+    this.logger.log(
+      `[log-only] would push "${n.title}" to ${tokens.length} device(s): ${n.body}`,
+    );
+    return { sent: 0, invalidTokens: [] };
+  }
+}