Smarter search cursor caching: track results_found, retry empty searches

- Added results_found column to search_cursors (migration 002)
- Exhausted + zero results: retry after 1 day (likely rate-limited)
- Exhausted + had results: stay cached 30 days (legitimately done)
- Not exhausted: standard 7-day staleness window
- Backfilled results_found from fetch_runs for existing cursors