Library page: new dropdown filter to show artists matched or unmatched
to any metadata source (Spotify, MusicBrainz, Deezer, Discogs, etc).
Select "No Discogs" to find artists needing manual Discogs matching.
Filter applied as WHERE clause on the source ID columns.
Discogs enrichment: added to valid_services whitelist, _enrichment_locks,
and _run_single_enrichment handler. The Enrich button was returning an
error when Discogs was selected from the dropdown.
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