修复企微服务热重载后丢失初始化的问题

get_wechat_service() 增加 _initialized 检查，
未初始化时自动调用 init(settings.wechat)，
避免 uvicorn 热重载后单例重建但未 init 导致 enabled=False。

app/services/wechat_service.py

@@ -32,6 +32,7 @@ class WeChatService:
 
     def __init__(self):
         self._enabled = False
+        self._initialized = False
         self._corp_id = ""
         self._agent_id = ""
         self._secret = ""
@@ -46,6 +47,7 @@ class WeChatService:
 
     def init(self, config):
         """初始化企微配置"""
+        self._initialized = True
         self._enabled = config.enabled and bool(config.corp_id) and bool(config.secret)
         self._corp_id = config.corp_id
         self._agent_id = config.agent_id
@@ -685,4 +687,7 @@ def get_wechat_service() -> WeChatService:
     global _wechat_service
     if _wechat_service is None:
         _wechat_service = WeChatService()
+    if not _wechat_service._initialized:
+        from app.config import settings
+        _wechat_service.init(settings.wechat)
     return _wechat_service