Make Spotify a selectable metadata source instead of auto-override
Spotify was automatically the primary metadata source whenever
authenticated, ignoring the user's fallback dropdown selection. Now:
- Dropdown renamed "Fallback Source" → "Primary Source" with Spotify
added as an option alongside iTunes, Deezer, Discogs
- User's selection drives search results, discovery page, status display
- Enhanced search uses selected primary source first, others as tabs
- Discovery page filters by selected source's artist IDs
- Spotify auth still works for playlists, followed artists, enrichment
- Default is 'deezer' — users who want Spotify select it explicitly
3c211eaac8