feat: multi-step cron — check-research + ingest refactor + pg_cron (#604)

New check-research cron route polls NotebookLM every 5min for docs in 'researching' status. Ingest refactored to fire-and-forget research (returns in ~15s). pg_cron SQL migration for all 4 schedules. Fixes: after() pattern, CRON_SECRET fail-closed auth, stuck detection independent of NotebookLM, researchData schema field.

Author: codercatdev