fix: SOCKS5 隧道修复 - 子线程事件循环兼容性

1. 去掉 asyncio.wait_for 包装 open_connection，改用直接调用
2. ConnectionHandler 增加 flask_app 参数，数据库操作使用 app context
3. InstanceManager 传递 flask_app 到 Socks5Server
4. 修复 _reply 中 writer.write 不需要 await 的问题

验证:
- 直连测试: HTTP/1.1 200 OK (ifconfig.me)
- 端口监听: 10800 正常工作

engine/instances.py

@@ -31,9 +31,10 @@ class InstanceConfig:
 class Socks5Server:
     """单个 SOCKS5 服务器的 asyncio 事件循环。"""
 
-    def __init__(self, config: InstanceConfig, user_service):
+    def __init__(self, config: InstanceConfig, user_service, flask_app=None):
         self.config = config
         self.user_service = user_service
+        self.flask_app = flask_app
         self.server = None
         self.loop = None
         self.thread = None
@@ -94,6 +95,7 @@ class Socks5Server:
         self._active_connections += 1
         handler = ConnectionHandler(
             reader, writer, self.config, self.user_service,
+            flask_app=self.flask_app,
             on_close_cb=self._on_connection_close
         )
         await handler.handle()
@@ -113,8 +115,9 @@ import time  # noqa: E402
 class InstanceManager:
     """管理所有 SOCKS5 实例。"""
 
-    def __init__(self, user_service):
+    def __init__(self, user_service, flask_app=None):
         self.user_service = user_service
+        self.flask_app = flask_app
         self._instances = {}       # name -> Socks5Server
         self._config_cache = {}    # name -> InstanceConfig
         self._lock = threading.Lock()
@@ -151,7 +154,7 @@ class InstanceManager:
         # 启动缺失的实例
         for name in current_names - running_names:
             cfg = self._config_cache[name]
-            srv = Socks5Server(cfg, self.user_service)
+            srv = Socks5Server(cfg, self.user_service, self.flask_app)
             srv.start()
             with self._lock:
                 self._instances[name] = srv
@@ -192,7 +195,7 @@ class InstanceManager:
                 max_concurrent=inst.max_concurrent,
             )
             self._config_cache[inst.name] = cfg
-            srv = Socks5Server(cfg, self.user_service)
+            srv = Socks5Server(cfg, self.user_service, self.flask_app)
             srv.start()
             with self._lock:
                 self._instances[inst.name] = srv