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

server/.env.example

@@ -19,3 +19,10 @@ REDIS_URL="redis://localhost:6379"
 # External APIs
 OPENWEATHER_API_KEY="your_openweather_api_key_here"
 # Optional: STORMGLASS_API_KEY="your_stormglass_api_key_here" (10 free requests/day for tide data)
+
+# Email Notifications (Mailgun)
+MAILGUN_API_KEY="dd27be7a81dd7b0980ada31fec9a9bd9-e80d8b76-fd80c63b"
+MAILGUN_DOMAIN="sandbox-123.mailgun.org"
+FROM_EMAIL="Beach Litter Platform <noreply@beachlitter.ie>"
+FRONTEND_URL="http://localhost:5173"
+TEST_EMAIL="test@example.com"