Backend: Complete notification delivery service (email + in-app)

[KuchiMercy]

Summary

The Notification entity, NotificationPreference entity, and notifications module structure exist, but the actual delivery pipeline is not wired up. Many features across the platform depend on notifications being delivered — this is the foundational backend service that unblocks notification UX.

Requirements

• Implement NotificationService with methods to create and deliver notifications
• Wire up Nodemailer SMTP integration for email delivery (already in dependencies)
• Create in-app notifications on all escrow lifecycle events:
  • ESCROW_FUNDED — notify all parties
  • MILESTONE_RELEASED — notify seller
  • DISPUTE_FILED — notify all parties + admins
  • DISPUTE_RESOLVED — notify all parties
  • ESCROW_EXPIRED — notify all parties
  • PARTY_INVITED — notify the invited party
  • CONDITION_MET — notify escrow creator
• Respect NotificationPreference settings (email on/off, in-app on/off per event type)
• Add endpoints: GET /notifications (list), PATCH /notifications/:id/read (mark read), GET /notifications/unread-count
• Add a NotificationPreference CRUD endpoint for users to manage their settings

Acceptance Criteria

• In-app notifications are created for all listed escrow events
• Email notifications are sent via SMTP when enabled in user preferences
• Users can fetch their notifications with pagination
• Unread count endpoint returns accurate count
• Mark as read updates the notification status
• Notification preferences are respected (no delivery if user opted out)
• Email delivery failures are logged but do not block in-app notification creation

Context

• Notification entity: apps/backend/src/notifications/notification.entity.ts
• Notification preference: apps/backend/src/notifications/notification-preference.entity.ts
• Notifications module: apps/backend/src/notifications/notifications.module.ts
• Nodemailer is already in package.json

Points: 150