Get Uptime Check Transcript

The call transcript for one check run — fetched lazily when a run's sidebar opens.

Never streams transcript bytes through the middleware (same as the eval/log transcript flow):

• primary runs: return a short-lived signed URL to the backing test_result's storage object, which the browser downloads directly.
• fallback runs: the turns already live inline on the check row (small JSONB, no storage object to sign), so return them normalized to the frontend TranscriptSegment shape. Exactly one of transcript_url / transcript is populated; both null when there's nothing.