[DevOps]: Make site-ingestion lock contention reliable

### Task Details

Split from #332. Ensure a staging run cannot silently cause the daily archive scan to be missed while preserving single-run access to ingestion state.

### Steps to Complete

- Define safe staging/archive lock-contention behavior.
- Ensure archive scans have a reliable non-overlapping window or safely wait for an active run.
- Make lock contention visible in job status and launcher logs.
- Add launcher tests for lock behavior.

### Additional Notes

- Preserve single-run serialization.
- Launcher: `backend/app/scripts/ingestion/sites/site_ingestion_launcher.sh`