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/admin/index.vue

@@ -1,5 +1,28 @@
 <template>
   <div class="container mx-auto p-6 max-w-6xl">
+    <!-- Maintenance Mode -->
+    <div class="alert mb-6" :class="maintenanceEnabled ? 'alert-warning' : 'alert-info'">
+      <div class="flex items-center justify-between w-full">
+        <div class="flex items-center gap-2">
+          <span class="font-medium">Mode maintenance</span>
+          <span v-if="maintenanceEnabled" class="badge badge-warning badge-sm">Actif</span>
+          <span v-else class="badge badge-ghost badge-sm">Inactif</span>
+        </div>
+        <button
+          class="btn btn-sm"
+          :class="maintenanceEnabled ? 'btn-ghost' : 'btn-warning'"
+          :disabled="maintenanceLoading"
+          @click="handleToggleMaintenance"
+        >
+          <span v-if="maintenanceLoading" class="loading loading-spinner loading-xs" />
+          {{ maintenanceEnabled ? 'Désactiver' : 'Activer' }}
+        </button>
+      </div>
+      <p class="text-sm opacity-70 mt-1">
+        {{ maintenanceEnabled ? 'Seuls les administrateurs peuvent accéder à l\'application.' : 'L\'application est accessible à tous les utilisateurs.' }}
+      </p>
+    </div>
+
     <div class="flex items-center justify-between mb-6">
       <h1 class="text-2xl font-bold">
         Administration des profils
@@ -153,9 +176,14 @@
 <script setup>
 import { ref, computed, onMounted } from 'vue'
 import DataTable from '~/components/common/DataTable.vue'
-import { useAdminProfiles } from '#imports'
+import { useAdminProfiles, useMaintenance } from '#imports'
 
 const { profiles, loading, fetchAll, createProfile, updateRole, setPassword, deactivateProfile } = useAdminProfiles()
+const { maintenanceEnabled, loading: maintenanceLoading, fetchStatus: fetchMaintenanceStatus, toggle: toggleMaintenance } = useMaintenance()
+
+const handleToggleMaintenance = async () => {
+  await toggleMaintenance()
+}
 
 const loaded = ref(false)
 const isLoading = computed(() => loading.value || !loaded.value)
@@ -264,7 +292,7 @@ const handleDeactivate = async (profileId) => {
 }
 
 onMounted(async () => {
-  await fetchAll()
+  await Promise.all([fetchAll(), fetchMaintenanceStatus()])
   loaded.value = true
 })
 </script>