fix(admin): use named export Cluster from ioredis 5.x

Previous `import { default as Cluster }` caused runtime to get Redis
instance instead of Cluster, resulting in ECONNREFUSED errors.

admin/src/lib/redis.ts

@@ -3,7 +3,7 @@
  * 支持单节点和集群模式
  * 前缀：admin:*
  */
-import Redis, { default as Cluster } from "ioredis";
+import Redis, { Cluster } from "ioredis";
 import type { ClusterNode } from "ioredis";
 import { REDIS_URL, REDIS_CLUSTER_URLS } from "./env";