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.
New POST /api/v1/request endpoint accepts a search query from external
sources (Discord bots, Home Assistant, curl) and triggers the
search-match-download pipeline asynchronously. Returns a request_id
for status polling via GET /api/v1/request/<id>. Optional notify_url
for callback on completion.
Also adds webhook_received trigger type and search_and_download action
type to the automation engine, so users can build custom flows like
"when webhook received → search & download → notify Discord".
Includes info panel in Settings showing endpoint URL and curl example.
