Inbound music requests are tracked in an in-memory _pending_requests
dict with a 1-hour TTL. Cleanup was only triggered inside
create_request(), so during quiet periods stale entries stayed in
memory until the next inbound request.
Add a background thread that wakes every 5 minutes and evicts any
entry older than _MAX_REQUEST_AGE. The thread is started once during
API blueprint registration (start_cleanup_thread is idempotent) and
is a daemon, so it exits automatically on process shutdown.
stop_cleanup_thread() is exposed for tests and future graceful-
shutdown hooks. It signals the stop event so the thread exits
without waiting for the next cleanup interval.
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