[Backend] Create unread notification count and mark-as-read API

[SYLVIANNORUKA]

Description: Implement API for retrieving unread notification counts and marking notifications as read.

Requirements:

• Handle unread count query requests
• Track read state per notification
• Validate ownership before marking as read
• Manage bulk mark-all-as-read requests
• Provide updated unread count after actions

Acceptance Criteria:

• Count query requests are handled
• Read state is tracked per notification
• Ownership is validated before marking read
• Bulk mark-all works
• Updated count is provided after actions

Technical Details:

Files: backend/src/modules/notifications/notifications.controller.ts, backend/src/modules/notifications/notifications.service.ts
Libraries: @nestjs/common, prisma