feat(notifications): add multi-channel notification module with Email, FCM, templates, and event listeners

- Domain: NotificationLog/NotificationPreference entities, repositories, channel value object
- Infrastructure: EmailService (nodemailer/SMTP), FcmService (firebase-admin), TemplateService (Handlebars)
- Application: SendNotification CQRS command, UserRegistered + AgentVerified event listeners
- Presentation: NotificationsController with history, preferences, and templates endpoints
- Prisma: NotificationLog and NotificationPreference models with proper indexes
- Templates: Vietnamese notification templates for user.registered, agent.verified, listing.approved, inquiry.received, password.reset

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

apps/api/src/modules/notifications/domain/events/notification-sent.event.ts

@@ -0,0 +1,14 @@
+import { type DomainEvent } from '@modules/shared/domain/domain-event';
+import { type NotificationChannel } from '../value-objects/notification-channel.vo';
+
+export class NotificationSentEvent implements DomainEvent {
+  readonly eventName = 'notification.sent';
+  readonly occurredAt = new Date();
+
+  constructor(
+    public readonly aggregateId: string,
+    public readonly userId: string,
+    public readonly channel: NotificationChannel,
+    public readonly templateKey: string,
+  ) {}
+}