Attach an already-uploaded audio file to the live transcript on a data
entry. The transcript node currently being written to via
appendLiveTranscriptUtterance or setLiveTranscript is wrapped in
an audio/video block carrying the supplied file_id, giving the
editor audio playback + transcript-sync.

Idempotent for the same file_id (returns 200). Returns 409 if the
transcript is already wrapped with a different file_id. Returns 404
if the data entry does not exist.