feat : add maintenance mode toggle in admin panel

- Backend: MaintenanceModeListener blocks non-admin API requests when
  var/maintenance flag file exists. MaintenanceController provides
  toggle (PUT /api/admin/maintenance) and public check endpoint
  (GET /api/maintenance/check).
- Frontend: Toggle button in admin page, maintenance.vue page for
  blocked users, middleware redirects non-admins to /maintenance.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Controller/MaintenanceController.php

@@ -0,0 +1,58 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Controller;
+
+use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
+use Symfony\Component\HttpFoundation\JsonResponse;
+use Symfony\Component\HttpKernel\KernelInterface;
+use Symfony\Component\Routing\Attribute\Route;
+
+final class MaintenanceController extends AbstractController
+{
+    public function __construct(
+        private readonly KernelInterface $kernel,
+    ) {}
+
+    #[Route('/api/maintenance/check', name: 'maintenance_check', methods: ['GET'])]
+    public function check(): JsonResponse
+    {
+        return new JsonResponse([
+            'enabled' => file_exists($this->flagPath()),
+        ]);
+    }
+
+    #[Route('/api/admin/maintenance', name: 'admin_maintenance_status', methods: ['GET'])]
+    public function status(): JsonResponse
+    {
+        $this->denyAccessUnlessGranted('ROLE_ADMIN');
+
+        return new JsonResponse([
+            'enabled' => file_exists($this->flagPath()),
+        ]);
+    }
+
+    #[Route('/api/admin/maintenance', name: 'admin_maintenance_toggle', methods: ['PUT'])]
+    public function toggle(): JsonResponse
+    {
+        $this->denyAccessUnlessGranted('ROLE_ADMIN');
+
+        $path = $this->flagPath();
+
+        if (file_exists($path)) {
+            unlink($path);
+            $enabled = false;
+        } else {
+            file_put_contents($path, (string) time());
+            $enabled = true;
+        }
+
+        return new JsonResponse(['enabled' => $enabled]);
+    }
+
+    private function flagPath(): string
+    {
+        return $this->kernel->getProjectDir() . '/var/maintenance';
+    }
+}