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

api/notifications_count.php

@@ -0,0 +1,29 @@
+<?php
+/**
+ * Notifications Count API (lightweight polling)
+ * 
+ * GET /api/notifications_count.php - Get unread count only
+ */
+
+require_once dirname(__DIR__) . '/f_core/config.boot.php';
+require_once dirname(__DIR__) . '/f_core/f_classes/class.notifications.php';
+
+header('Content-Type: application/json');
+
+// Require authentication
+session_start();
+if (empty($_SESSION['user_id'])) {
+    http_response_code(401);
+    echo json_encode(['success' => false, 'error' => 'Authentication required']);
+    exit;
+}
+
+$usr_id = $_SESSION['user_id'];
+$notifications = new VNotifications();
+
+$count = $notifications->getUnreadCount($usr_id);
+
+echo json_encode([
+    'success' => true,
+    'unread_count' => $count
+]);