New "Scan Lookback" dropdown in the watchlist artist config modal.
Each artist can override the global lookback period (7d to entire
discography). Default is "Use Global Setting" (NULL in DB).
- Database: lookback_days INTEGER DEFAULT NULL on watchlist_artists,
auto-migrated on startup
- Scanner: checks per-artist lookback_days first, falls back to
global discovery_lookback_period if NULL
- Backend: GET/POST /api/watchlist/artist/<id>/config includes
lookback_days. Changing lookback clears last_scan_timestamp to
force a rescan with the new window
- Frontend: dropdown with 8 options in artist config modal
- Fully backwards compatible — existing artists unchanged
a33f891fa6
