feat: Add manual event notification endpoint for admins

- POST /api/events/:eventId/notify - send notifications from admin's email
- sendSingleEventNotification() uses admin's primary email account
- getSenderAccountByUserId() to get admin's email credentials

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/cron/index.js

@@ -1,5 +1,5 @@
 import { logger } from '../utils/logger.js';
-import { startCalendarNotificationCron, triggerEventNotifications } from './calendar/index.js';
+import { startCalendarNotificationCron, triggerEventNotifications, triggerSingleEventNotification } from './calendar/index.js';
 
 /**
  * Start all cron jobs
@@ -14,4 +14,4 @@ export const startAllCronJobs = () => {
 };
 
 // Export individual functions for testing/manual triggers
-export { triggerEventNotifications };
+export { triggerEventNotifications, triggerSingleEventNotification };