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/src/modules/metrics/metrics.module.ts

@@ -15,6 +15,8 @@ import {
   DB_QUERY_DURATION,
   DB_POOL_ACTIVE_CONNECTIONS,
   SEARCH_QUERY_DURATION,
+  GOODGO_WS_CONNECTED_CLIENTS,
+  GOODGO_WS_MESSAGES_TOTAL,
   WEB_VITALS_LCP,
   WEB_VITALS_FCP,
   WEB_VITALS_CLS,
@@ -83,6 +85,18 @@ import { HttpMetricsInterceptor } from './presentation/interceptors/http-metrics
       labelNames: ['plan'],
     }),
 
+    // ── WebSocket Metrics ──
+    makeGaugeProvider({
+      name: GOODGO_WS_CONNECTED_CLIENTS,
+      help: 'Number of active WebSocket clients',
+      labelNames: ['namespace'],
+    }),
+    makeCounterProvider({
+      name: GOODGO_WS_MESSAGES_TOTAL,
+      help: 'Total number of WebSocket messages emitted/received',
+      labelNames: ['namespace', 'event', 'direction'],
+    }),
+
     // ── Services & Interceptors ──
     MetricsService,
     HttpMetricsInterceptor,