Upload pushed playback audio

Send the raw audio body for a previously accepted Session Playback with type: push. Use the URL, method, and headers returned in the Playback response, and also send the normal Authorization: Bearer <app_uuid>:<api_key> header. The operation must match the accepted Playback, remain active, and the one-shot request must be opened before ingest.expires_at; an accepted request may continue streaming afterward. Send exactly one unframed stream and close the request body at EOF. A 204 means the complete request body was accepted; playback completion is reported separately by lifecycle webhooks.