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 maintain the Mercure protocol (built on SSE) and the reference implementation (written in Go, available as a standalone binary and a Caddy module) which does exactly that: https://mercure.rocks
In addition to the free and open source server, we also provide a cloud offering and on-premises versions that support clustering using Redis Streams, Kafka, Pulsar or Postgres LISTEN/NOTIFY as backends.
The solution is used by many big actors in production for years:
I've come across https://github.com/soketi/soketi and https://centrifugal.dev/ but not sure if there are more battle-tested solutions.