feat: Add comprehensive notification system with email support

- Create notification management system with VNotifications class
- Build notification API endpoints (get, mark read, delete, preferences)
- Implement notification bell UI widget with real-time polling
- Add email queue system with digest support (instant/hourly/daily/weekly)
- Create notification trigger helpers for common events
- Add fine-grained user preference controls
- Implement cron script for email processing and cleanup
- Document setup and usage in docs/NOTIFICATIONS.md

Features:
- In-app notifications with dropdown
- Email delivery with batching
- 6 notification types (comment, like, subscribe, upload, mention, system)
- User preference controls per notification type
- Lightweight polling (30s interval)
- Auto-cleanup of old notifications (90 days)
- Production-ready with proper error handling

app_scripts/cron/process_notification_emails.php

@@ -0,0 +1,25 @@
+<?php
+/**
+ * Process Notification Email Queue
+ * Run via cron every 5-15 minutes
+ */
+
+require_once dirname(__DIR__, 2) . '/f_core/config.boot.php';
+require_once dirname(__DIR__, 2) . '/f_core/f_classes/class.notifications.php';
+require_once dirname(__DIR__, 2) . '/f_core/f_classes/class.logger.php';
+
+$logger = new VLogger('email_queue');
+$notifications = new VNotifications();
+
+$logger->info("Starting email queue processing");
+
+$sent = $notifications->processEmailQueue(50);
+
+$logger->info("Email queue processing complete. Sent: $sent emails");
+
+// Clean up old notifications (older than 90 days)
+$deleted = $notifications->cleanup(90);
+
+$logger->info("Cleaned up $deleted old notifications");
+
+echo "Done. Sent: $sent, Cleaned: $deleted\n";