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/notifications/notifications.module.ts

@@ -1,6 +1,7 @@
 import { Module } from '@nestjs/common';
 import { CqrsModule } from '@nestjs/cqrs';
 import { AuthModule } from '@modules/auth';
+import { MetricsModule } from '@modules/metrics';
 import { SendNotificationHandler } from './application/commands/send-notification/send-notification.handler';
 import { AgentVerifiedListener } from './application/listeners/agent-verified.listener';
 import { EmailChangeRequestedListener } from './application/listeners/email-change-requested.listener';
@@ -53,7 +54,7 @@ const EventListeners = [
 ];
 
 @Module({
-  imports: [CqrsModule, AuthModule],
+  imports: [CqrsModule, AuthModule, MetricsModule],
   controllers: [NotificationsController, ZaloOaWebhookController],
   providers: [
     // Repositories