refactor: implement middleware chain (#4240)

* refactor: implement middleware chain

* refactor

middlewares/MaintenanceMiddleware.php

@@ -0,0 +1,17 @@
+<?php
+
+declare(strict_types=1);
+
+class MaintenanceMiddleware implements Middleware
+{
+    public function __invoke(Request $request, $next): Response
+    {
+        if (!Configuration::getConfig('system', 'enable_maintenance_mode')) {
+            return $next($request);
+        }
+        return new Response(render(__DIR__ . '/../templates/error.html.php', [
+            'title' => '503 Service Unavailable',
+            'message' => 'RSS-Bridge is down for maintenance.',
+        ]), 503);
+    }
+}