Replaced single "Change" button with per-source rows showing match
status for each provider (Spotify, Apple Music, Deezer, Discogs).
Each row has Fix/Match button that searches that specific source API,
plus clear button to remove individual matches.
- Per-source search uses _search_service (same as enrichment modal)
- Backend: added Discogs to valid providers, empty ID clears match
- Fixed provider validation to accept 'discogs' alongside others
- Clear sets DB column to NULL instead of rejecting empty string
7a24431e46