Graceful shutdown doesn't signal open streams to close

When this instance is asked to shut down -- a rolling deploy, docker compose down, a Kubernetes pod eviction -- every currently-open notification stream connection just keeps running exactly as before, right up until the ASGI server forcibly tears the connection down out from under it, which either happens abruptly after a grace period or not at all within a reasonable window. Every rolling deploy leaves connected clients hanging instead of reconnecting promptly against whichever instance takes over next.

app/main.py's lifespan already creates app.state.shutdown_event as an asyncio.Event on startup, and app/routers/stream.py already passes it through to event_stream() as the shutdown argument -- the plumbing is there -- but the lifespan's shutdown path never calls .set() on it, and event_stream()'s main loop never reads it even though it's sitting right there as a parameter.

1. pytest practice_tickets/tests/test_ticket_15_graceful_shutdown_signal.py -v
2. Observe two failures: a live event_stream() generator ignores a set shutdown Event and keeps emitting ordinary heartbeats, and a real lifespan exit never actually sets app.state.shutdown_event in the first place.

This is the shutdown-time counterpart to the disconnect check event_stream()'s main loop already does for the client going away -- the same loop needs an equivalent check for the server going away. Both need to be checked on every iteration, not just once at the top of the function, since either one can happen at any point during a long-lived connection's life. And the shutdown side needs to happen exactly once, cleanly, regardless of how many connections are currently open -- one asyncio.Event, set once, is what every open generator's loop should be watching.

app/main.py's lifespan finally block is where shutdown begins -- that's where app.state.shutdown_event.set() needs to happen, before or as Redis gets closed. event_stream()'s while True loop already checks request.is_disconnected() on every iteration; the shutdown parameter it already accepts needs the equivalent check sitting next to it, returning cleanly when it's set.