Hand back the user's own recording, for as long as it is kept.

Meant for a take that failed: "your recording is safe" has to be true, and this is what makes it checkable. A transcribed take has no recording left, since only the transcript is kept. A failed take's recording is deleted within 48 hours (see retention.py), and one that is due is refused even before the sweep reaches it; a signed URL for one that is gone would only fail at the bucket, so say so here instead.