feat: implement notification system with Mailgun email integration

- Add notifications table with migration
- Create NotificationService with area-of-interest matching
- Integrate Mailgun for email delivery
- Add notification API endpoints (get, mark as read, unread count)
- Create scheduled jobs for event reminders and cleanup
- Build NotificationBell component for frontend
- Add notification triggers for reports and events
- Implement comprehensive test script
- Update Header with notification bell
- Add email templates with HTML formatting

Requirements satisfied:
- 10.1: Users can define areas of interest (1-50km radius)
- 10.2: Notifications sent within 10 minutes
- 10.3: Notifications for new reports/events in user area
- 10.4: Configurable notification preferences (email/in-app)

Task 33 completed

.kiro/specs/beach-litter-mapping/tasks.md

@@ -277,7 +277,7 @@
   - Implement 6-hour cache expiration
   - _Requirements: 11.1, 11.2, 11.3, 11.4, 11.5, 11.6_
 
-- [ ] 33. Implement notification system
+- [x] 33. Implement notification system
   - Create notification service with area-of-interest matching
   - Build email notification integration (SendGrid)
   - Implement in-app notification storage and retrieval