Two complementary fixes to stop distinct artists ending up with the same source
id (the near-name collisions: ODESZA/odessa, Blance/Blanke, Lady A/Lady Gaga,
plus MusicBrainz's combined-score weak matches like Grant/Amy Grant):
- core/worker_utils.accept_artist_match() / source_id_conflict(): one shared,
tested gate. Rejects artist matches below 0.85 (stricter than the 0.80 used
for album/track titles, since short artist names false-positive easily) AND
refuses to store a source id a DIFFERENTLY-named artist already holds. A
same-named holder (one act across two media servers) is still allowed.
- Routed every artist-match worker through it: deezer, qobuz, tidal, discogs,
itunes, spotify (its scorer now uses the 0.85 threshold), audiodb, and
musicbrainz (conflict guard only — its matcher is combined-score, so the
guard is the net that catches its weak-name matches).
Centralizing in worker_utils avoids the copy-paste that let the original
album/track overwrite bug live in four workers at once. 17 new gate tests.
