after you delete a file, the scans dedup against download history so it won't
re-grab — this gives you the escape hatch:
- per-row 'Re-download' button in the History modal: forgets that grab (DELETE
/downloads/history/<id>) so the next scan re-adds + re-downloads it.
- 'Clear' button: wipes the whole history (guarded confirm).
db.delete_download_history / clear_download_history(kind?) + the two endpoints.
tested (db methods + endpoints).
