feat(router): 添加用户追踪和节点通知功能

在ROUTER_MODE启用时跟踪用户消息，并在节点注册/注销时通知相关用户。新增_known_users集合记录活跃用户，重构通知逻辑以支持所有已知用户或特定服务用户的通知。

bot/handler.py

@@ -129,6 +129,11 @@ async def _process_message(user_id: str, chat_id: str, text: str) -> None:
             await send_text(chat_id, "chat_id", "Sorry, you are not authorized to use this bot.")
             return
 
+        from config import ROUTER_MODE
+        if ROUTER_MODE:
+            from router.nodes import get_node_registry
+            get_node_registry().track_user(user_id)
+
         reply = await handle_command(user_id, text)
         if reply is not None:
             if reply: