feat: track notification read receipts per user

- New notification_reads table (migration 008) with unique (notification_id, user_id)
- Notification model: markAsRead inserts into notification_reads; getForUser JOINs
  to show per-user read status; getUnreadCount checks NOT EXISTS in reads table
- getAll() includes read_count subquery for admin view
- Admin view shows 'X / Y' read count for broadcast notifications,
  'Read'/'Unread' badge for user-targeted ones
- Migrates existing read notifications into notification_reads
- AGENTS.md updated with new conventions

src/Controllers/AdminController.php

@@ -255,10 +255,13 @@ class AdminController
         $userModel = new User();
         $users = $userModel->getAll(1, 200);
 
+        $totalActiveUsers = $notificationModel->getTotalActiveUsers();
+
         $this->view->display('admin.notifications', [
             'title' => 'Notifications - ServerManager',
             'notifications' => $allNotifications,
             'users' => $users,
+            'totalActiveUsers' => $totalActiveUsers,
         ]);
     }