Operator trigger: render audio for up to limit published briefings that are missing audio_keys.

Useful for one-shot backfill (e.g. limit=200 drains all 145 existing rows in a single call) and for testing without waiting for the 10-min worker interval. The worker itself runs the same function on schedule so this endpoint adds zero unique logic — it's just an on-demand entry point.

Auth: admin token (X-Storyflo-Email-Token).