fix(deploy): 适配宁波 NGINX 配置加载路径

宁波 NGINX 仅加载 conf.d/*.conf，原切换脚本写入 sites-enabled 后不会生效。统一将维护页、集群入口和回滚配置写入实际加载的 conf.d 入口，并保留切换前配置备份。\n\n验证：bash -n、ShellCheck、git diff --check、双节点 nginx -t 与 prepare。

scripts/cluster/install-nginx-edges.sh

@@ -81,11 +81,10 @@ if [[ $mode == activate ]]; then
   for host in "$CLUSTER_NINGBO_HOST" "$CLUSTER_HONGKONG_HOST"; do
     cluster_ssh "$host" 'bash -s' <<'REMOTE'
 set -euo pipefail
-ln -sfn /etc/nginx/sites-available/ai.51easyai.com-cluster \
+install -m 0644 /etc/nginx/sites-available/ai.51easyai.com-cluster \
+  /etc/nginx/conf.d/easyai-ai-gateway-ai.51easyai.com.conf
+rm -f /etc/nginx/sites-enabled/ai.51easyai.com \
   /etc/nginx/sites-enabled/ai.51easyai.com-cluster
-if [[ -L /etc/nginx/sites-enabled/ai.51easyai.com ]]; then
-  rm -f /etc/nginx/sites-enabled/ai.51easyai.com
-fi
 nginx -t
 systemctl reload nginx
 REMOTE