feat(proxy): worker 退出时优雅关闭 tunnel 连接，提升 max-requests 默认值

- TunnelManager 新增 shutdown_all 方法，drain 飞行中请求后发送 GoAway
- shutdown 期间标记 draining 拒绝新请求进入
- gunicorn max-requests 默认值从 4000 提升到 50000，减少不必要的 worker 重启

src/modules/proxy_nodes/__init__.py

@@ -101,14 +101,19 @@ async def _on_startup() -> None:
 
 
 async def _on_shutdown() -> None:
-    """停止心跳检测调度器"""
+    """优雅关闭 tunnel 连接并停止心跳检测调度器"""
     import logging
 
     from src.services.proxy_node.health_scheduler import get_proxy_node_health_scheduler
+    from src.services.proxy_node.tunnel_manager import get_tunnel_manager
     from src.utils.task_coordinator import StartupTaskCoordinator
 
     logger = logging.getLogger("aether.modules.proxy_nodes")
 
+    # 先向所有 tunnel 连接发送 GoAway，让 proxy 端立即重连到其他 worker
+    manager = get_tunnel_manager()
+    await manager.shutdown_all()
+
     from src.clients import get_redis_client
 
     redis_client = await get_redis_client()