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/composables/useMaintenance.ts

@@ -0,0 +1,30 @@
+import { ref } from 'vue'
+import { useApi } from './useApi'
+
+const maintenanceEnabled = ref(false)
+
+export function useMaintenance() {
+    const { apiCall } = useApi()
+    const loading = ref(false)
+
+    const fetchStatus = async () => {
+        const res = await apiCall<{ enabled: boolean }>('/admin/maintenance')
+        if (res.success && res.data) {
+            maintenanceEnabled.value = res.data.enabled
+        }
+    }
+
+    const toggle = async () => {
+        loading.value = true
+        try {
+            const res = await apiCall<{ enabled: boolean }>('/admin/maintenance', { method: 'PUT' })
+            if (res.success && res.data) {
+                maintenanceEnabled.value = res.data.enabled
+            }
+        } finally {
+            loading.value = false
+        }
+    }
+
+    return { maintenanceEnabled, loading, fetchStatus, toggle }
+}