refactor: use laravel notifications for system user private messages

app/Interfaces/SystemNotificationInterface.php

@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * NOTICE OF LICENSE.
+ *
+ * UNIT3D Community Edition is open-sourced software licensed under the GNU Affero General Public License v3.0
+ * The details is bundled with this project in the file LICENSE.txt.
+ *
+ * @project    UNIT3D Community Edition
+ *
+ * @author     Roardom <roardom@protonmail.com>
+ * @license    https://www.gnu.org/licenses/agpl-3.0.en.html/ GNU Affero General Public License v3.0
+ */
+
+namespace App\Interfaces;
+
+use App\Models\User;
+
+interface SystemNotificationInterface
+{
+    /**
+     * @return array{subject: string, message: string}
+     */
+    public function toSystemNotification(User $notifiable): array;
+}