feat(节点跟踪): 改进用户跟踪功能并添加新用户通知

当新用户首次发送消息时，返回布尔值并通知可用节点。在handler.py中添加对新用户的处理逻辑，当检测到新用户时，向其发送当前在线的节点列表。这有助于新用户更快了解可用资源。

router/nodes.py

@@ -69,9 +69,12 @@ class NodeRegistry:
             return True
         return secret == self._secret
 
-    def track_user(self, user_id: str) -> None:
-        """Record a user as known (has sent at least one message)."""
+    def track_user(self, user_id: str) -> bool:
+        """Record a user as known. Returns True if this is their first message."""
+        if user_id in self._known_users:
+            return False
         self._known_users.add(user_id)
+        return True
 
     def _get_notifiable_users(self, node: NodeConnection) -> Set[str]:
         """Get users to notify about a node event.