One SSE connection, N upstream agents — the stream relay we shipped

The pod that was tailing one of our long-running agents got rotated during a deploy. The client connection to the dashboard didn't disconnect — it sat there, EventSource happily believing the world was fine, with zero events coming through. The agent was still working. The events were still being emitted upstream. The middle box that was supposed to forward them to the browser was simply gone. That moment is the reason this post exists. If you're going to put a relay between N independent event-