feat: unified cron pipeline — replaces webhooks with single cron driver

The check-renders cron now handles ALL pipeline status transitions:
- script_ready → audio_gen (claims, then runs video pipeline via after())
- rendering → video_gen (checks Lambda progress, downloads, uploads)
- video_gen → uploading → published (claims, then runs distribution via after())
- Stuck detection: audio_gen >10min, rendering >30min → flagged

"Claim first, work second" pattern prevents duplicate processing
on overlapping 1-2 minute cron runs. Sanity webhooks removed.

Author: Miriad