feat: multi-step cron — check-research route + ingest refactor + pg_cron SQL

- NEW: app/api/cron/check-research/route.ts (690 lines)
  Polls NotebookLM research status every 5min. When complete:
  imports sources, generates 5 infographics, enriches script, transitions to script_ready.
  Includes stuck detection (>30min → flagged).

- MODIFIED: app/api/cron/ingest/route.ts
  Fire-and-forget research: creates notebook + starts research in ~15s,
  creates doc as "researching". Returns immediately.
  Non-research path unchanged (straight to script_ready).

- MODIFIED: sanity/schemas/documents/automatedVideo.ts
  Added "researching" status, researchNotebookId, researchTaskId,
  trendScore, trendSources fields.

- NEW: supabase/migrations/002_cron_schedules.sql
  pg_cron schedules for all 4 pipeline crons.

Author: codercatdev