Handle Recording Status Callback

Twilio webhook: fires when the dual-channel recording we started on the carrier leg is finalized (BYOB attended transfer, or a Vapi-native warm transfer on vapi_transfers_enabled).

By the time Twilio fires RecordingStatus=completed, the carrier call has hung up, every Vapi end-of-call-report has been delivered, and the full audio file is available. That makes this the right moment to trigger the transcription pipeline for these calls — earlier triggers (the Vapi end-of-call-report) fire while the carrier call is still in progress and would capture only the pre-transfer half.

absent/failed mean no usable Twilio file exists. Those clear call_sid so the pipeline falls back to Vapi's own recording, then trigger — dropping them would leave the call with no transcript at all.

No authentication (Twilio webhook).