feat(notifications): production-ready WebSocket gateway (TEC-2766)

- Add RedisIoAdapter (shared/infra) for multi-instance Socket.IO fan-out
  with graceful fallback to the in-memory IoAdapter when Redis is
  unreachable.
- Pin Socket.IO heartbeat (pingInterval/pingTimeout/connectTimeout)
  via env-tunable gateway options for reconnect stability.
- Expose Prometheus metrics on /notifications: goodgo_ws_connected_clients
  (Gauge) and goodgo_ws_messages_total (Counter) with namespace/event/
  direction labels. Wired through MetricsService and tracked across
  connect/disconnect + emits.
- Unit tests: RedisIoAdapter connect/fallback/close, new MetricsService
  WS helpers, and gateway metric increments/decrements on auth paths.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/package.json

@@ -37,6 +37,7 @@
     "@prisma/client": "^7.7.0",
     "@sentry/nestjs": "^10.47.0",
     "@sentry/profiling-node": "^10.47.0",
+    "@socket.io/redis-adapter": "^8.3.0",
     "@willsoto/nestjs-prometheus": "^6.1.0",
     "bcrypt": "^6.0.0",
     "bullmq": "^5.74.1",