feat Notifications and Occupancy routes.

[AffableMelon]

• Notifications: Added routes for creating, fetching, updating, and deleting notifications, with WebSocket support for real-time updates. (backend/controllers/notification.js, backend/controllers/notification.jsR1-R143)
• Occupancy: Added routes for managing classroom occupancy, including conflict detection for overlapping time slots. (backend/controllers/occupancy.js, backend/controllers/occupancy.jsR1-R237)