refactor(proxy): 移除 unhealthy 状态、前端展示失败率替换连接数、命令提示修正

- 移除 ProxyNodeStatus.UNHEALTHY 枚举值，仅保留 online/offline
- 迁移脚本将已有 unhealthy 数据迁移为 offline，upgrade/downgrade 均有幂等性保护
- 前端代理节点列表用失败率列替换连接数列，超过 5% 高亮显示
- 节点列表排序从 updated_at DESC 改为 name ASC
- Rust 端命令行提示从 aether-proxy 改为 ./aether-proxy

src/api/admin/proxy_nodes/routes.py

@@ -172,7 +172,7 @@ async def unregister_proxy_node(request: Request, db: Session = Depends(get_db))
 @router.get("")
 async def list_proxy_nodes(
     request: Request,
-    status: str | None = Query(None, description="按状态筛选：online/unhealthy/offline"),
+    status: str | None = Query(None, description="按状态筛选：online/offline"),
     skip: int = Query(0, ge=0),
     limit: int = Query(100, ge=1, le=1000),
     db: Session = Depends(get_db),