docs: add Swagger documentation for notification routes

[digrajsaini100]

Summary

• Added Swagger/OpenAPI documentation for the notification service health endpoint.
• Added Swagger/OpenAPI documentation for retrieving notifications.
• Documented successful and error response structures.
• Grouped both endpoints under the Notifications tag.

Documented Endpoints

• GET /api/notifications/health
• GET /api/notifications

Testing

• Documentation-only change.
• Local runtime and Docker testing were not performed.

[s222530306]

@digrajsaini100 Please ensure that you are requesting PR reviews from @trungquantrannguyen, as they are the Backend team leader.