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/main.ts

@@ -8,11 +8,10 @@ import './instrument';
 
 import { RequestMethod, ValidationPipe } from '@nestjs/common';
 import { NestFactory } from '@nestjs/core';
-import { IoAdapter } from '@nestjs/platform-socket.io';
 import { DocumentBuilder, SwaggerModule } from '@nestjs/swagger';
 import cookieParser from 'cookie-parser';
 import helmet from 'helmet';
-import { LoggerService, validateEnv } from '@modules/shared';
+import { LoggerService, RedisIoAdapter, validateEnv } from '@modules/shared';
 import { AppModule } from './app.module';
 
 async function bootstrap() {
@@ -60,7 +59,11 @@ async function bootstrap() {
   });
 
   // ── WebSocket Adapter (Socket.IO) ──
-  app.useWebSocketAdapter(new IoAdapter(app));
+  // Redis pub/sub fan-out for multi-instance broadcasts; falls back to the
+  // in-memory IoAdapter when Redis is unreachable (single-node / local dev).
+  const wsAdapter = new RedisIoAdapter(app);
+  await wsAdapter.connectToRedis();
+  app.useWebSocketAdapter(wsAdapter);
 
   // ── Security Headers (Helmet) ──
   app.use(