centrifugal/centrifugo ? reverse-engineered prompt

I want to build a self-hosted real-time messaging server, basically my own version of a service like Pusher or Ably that I can run myself. The main idea is that my backend application can send a message to this server, and the server will instantly push that message to all connected users. This will be the foundation for features like live comments, notifications, chat applications, or real-time data dashboards.

It needs to be highly scalable to handle a lot of concurrent users over websockets and similar technologies. It's important that it's language-agnostic, so I can use it with my existing backend, whatever language it's in. Please include features like secure user authentication and channel permissions. If a user disconnects and reconnects, they should get the messages they missed. A nice-to-have would be a built-in admin web dashboard to see what's going on with connections and channels.