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>

frontend/app/pages/maintenance.vue

@@ -0,0 +1,21 @@
+<template>
+  <div class="min-h-screen flex items-center justify-center bg-base-200">
+    <div class="text-center max-w-md">
+      <h1 class="text-4xl font-bold mb-4">
+        Maintenance
+      </h1>
+      <p class="text-lg text-base-content/70 mb-6">
+        L'application est actuellement en maintenance. Veuillez réessayer ultérieurement.
+      </p>
+      <button class="btn btn-primary" @click="retry">
+        Réessayer
+      </button>
+    </div>
+  </div>
+</template>
+
+<script setup>
+const retry = () => {
+  navigateTo('/')
+}
+</script>