Stop the active stream for a conversation and release its lock.

Two things happen, in order:

1. Signal the worker to stop generating (stop:{conv} list → BLPOP in the worker's stop_listener).
2. Release the stream lock so the user's follow-up message doesn't 409 while the worker is still unwinding — it can't interrupt mid-tool-call and still drains background hydration before its finally, so the lock could otherwise be held for seconds after the user clicked Stop.

Both are gated on an actually-active stream, so we never plant a stale stop: signal that a future turn's worker would consume. We signal before clearing the lock so the current worker's already-blocked BLPOP wins the signal ahead of any (later-starting) follow-up worker.