refactor: move backend in apps/backend

apps/backend/bootstrap/providers.php

@@ -0,0 +1,7 @@
+<?php
+
+return [
+    \apps\backend\app\Providers\AppServiceProvider::class,
+    \apps\backend\app\Providers\BroadcastServiceProvider::class,
+    \apps\backend\app\Providers\GateServiceProvider::class,
+];