- Add similar artists fetching to web UI scan loop
- Add database migration for UNIQUE constraint on similar_artists table - Add source-agnostic /api/discover/album endpoint for iTunes support
- Fix NOT NULL constraint on discovery_recent_albums blocking iTunes albums
- Add fallback to watchlist artists when no similar artists exist
- Add /api/discover/refresh and /api/discover/diagnose endpoints
- Add retry logic with exponential backoff for iTunes API calls
- Ensure cache_discovery_recent_albums runs even when pool population skips
