feat: add notification priority and grouping system

[gavin913427-hash]

Changes

Implement notification system with priority levels and groups.

Features

• Notification model with priority (low/medium/high/urgent) and groups (bills/expenses/budget/security/system)
• NotificationService with CRUD, filtering, pagination
• REST API: GET/POST/PATCH/DELETE /notifications
• Mark single/all notifications as read (with group scope)
• Unread count by group
• Action URLs and JSON metadata support

Files Modified

• packages/backend/app/models.py - Added Notification model, enums
• packages/backend/app/routes/init.py - Registered notifications blueprint
• packages/backend/tests/conftest.py - Fixed test environment with fakeredis
• packages/backend/app/routes/notifications.py - NEW: REST API
• packages/backend/app/services/notifications.py - NEW: NotificationService
• packages/backend/app/services/README_NOTIFICATIONS.md - NEW: Documentation
• packages/backend/tests/test_notifications.py - NEW: 8 tests

Fixes #122