feat: add check-research cron route for polling NotebookLM

New cron route that polls docs in "researching" status:
- Queries Sanity for docs with researchNotebookId
- Polls NotebookLM research status
- On completion: imports sources, generates infographics,
  gets summary, re-generates enriched Gemini script, runs critic
- Updates Sanity doc to "script_ready" with enriched data
- Stuck detection: flags docs >30min in "researching"
- Follows check-renders patterns (auth, fetchCache, maxDuration)

Co-authored-by: research <research@miriad.systems>

Author: Miriad