feat(notifications): add Zalo OA webhook controller + WebSocket gateway tests

- Add ZaloOaWebhookController: GET verification endpoint, POST event handler
  for follow/unfollow/user_send_text events with user linking via OAuthAccount
- Register webhook controller in NotificationsModule
- Add 13 unit tests for webhook (challenge verify, follow/unfollow/message
  handling, linked/unlinked users, error resilience)
- Add 18 unit tests for NotificationsGateway (JWT auth, multi-device tracking,
  disconnect cleanup, notification.sent event, Redis cache, unread count)

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

apps/api/src/modules/notifications/notifications.module.ts

@@ -27,6 +27,7 @@ import { StringeeSmsService } from './infrastructure/services/stringee-sms.servi
 import { TemplateService } from './infrastructure/services/template.service';
 import { ZaloOaService } from './infrastructure/services/zalo-oa.service';
 import { NotificationsController } from './presentation/controllers/notifications.controller';
+import { ZaloOaWebhookController } from './presentation/controllers/zalo-oa-webhook.controller';
 import { NotificationsGateway } from './presentation/gateways/notifications.gateway';
 
 const CommandHandlers = [SendNotificationHandler];
@@ -51,7 +52,7 @@ const EventListeners = [
 
 @Module({
   imports: [CqrsModule, AuthModule],
-  controllers: [NotificationsController],
+  controllers: [NotificationsController, ZaloOaWebhookController],
   providers: [
     // Repositories
     { provide: NOTIFICATION_REPOSITORY, useClass: PrismaNotificationRepository },