Get the post-call merged transcript

The post-call merged transcript: both sides of the call on one timeline, built from the dual-channel recording after the call ends. This is a different, fuller record than the live transcript_full rows, not a tidied-up copy of them - the recording captures the callee's channel in full, so the merged transcript routinely contains speech the live pipeline never transcribed: IVR phone trees, hold and queue announcements, anything said before the STT gate opened. If a recording plainly contains something your transcript doesn't, this endpoint is where to look.