Is there a modern open-source solution for bridging a traditional stateless web application to real-time notifications - one that's implemented all the best practices from the OP? Something like pusher.com but on self-hosted infrastructure/k8s, where messages from clients are turned into webhooks to an arbitrary server, and the server can HTTP POST to a public/private channel that clients can subscribe to if they know the channel secret.
I've come across https://github.com/soketi/soketi and https://centrifugal.dev/ but not sure if there are more battle-tested solutions.