feat(notifications): persistent student notifications with per-student clear & email targeting

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Summary

This PR introduces persistent student notifications in MongoDB, real-time WebSocket distribution, per-student notification management (clearing), CSV target email filtering, and reliable notification bell ringing indicators across page navigation and reconnects.

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Key Changes

Backend

• Notification Model (backend/src/models/Notification.js): Created Mongoose model storing roomCode, announcement, target emails list, fileName, clearedBy (array of student User ObjectIds), and createdAt.
• Notification API Routes (backend/src/routes/notifications.js):
  • GET /api/notifications: Fetches active, uncleared notifications matching the student's email or targeted to all.
  • POST /api/notifications/clear: Marks notifications as cleared strictly for the requesting student by appending their User ID to clearedBy.
• WebSocket Socket Server (backend/src/index.js): Updated new_announcement handler to save the announcement to MongoDB (Notification.create) upon emission while keeping real-time socket broadcasting.

Frontend

• Student Dashboard (frontend/src/pages/StudentDashboard.jsx):
  • Automatically fetches persistent uncleared notifications from MongoDB on mount/reload.
  • Updated "Clear all" and individual ✕ dismiss actions to update backend state so clearing is individual to each student.
  • Triggers the bell ringing animation whenever uncleared notifications exist.
• Global Socket Store (frontend/src/stores/socketStore.js): Added a global new_announcement listener so students receive and record incoming notifications regardless of which page/route they are currently on.

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Verification & Behavior

• Target Email Filtering: Sockets and database queries verify student emails against uploaded CSV lists; non-targeted students will not see or receive specific announcements.
• Per-Student Clear: Student A clearing their notifications adds Student A's ID to clearedBy without removing the notification for Student B.
• Reliable Bell Indicator: The notification bell rings dynamically on the dashboard even if the student was offline or on another page when the announcement was created.