Receive a LiveKit server webhook event

Receives webhook events from the LiveKit server (e.g. participant_left, participant_connection_aborted, track_unpublished), used as a last-resort signal for participant disconnections not caught by healthchecks or client-side events. LiveKit broadcasts each event to every configured worker URL, so this worker verifies the request signature and silently ignores events that don't belong to a participant/loadtest run it created. Always responds 200 once the request body has been read, regardless of whether the event was recognized, verified, or acted upon.