fix: ListenBrainz similarity is stored relative to the seed's best, and the candidate arms keep tier order (#6177)

LB's similarity scores are counts (16 to 1445 for recordings, to 10,875
for artists), and they reached a scorer that reads similarity as 0 to 1.
One edge added 64 to 5,800 points under Songs-like, so likes, skips and
recency stopped mattering for any candidate with an edge.

- ResolveListenBrainzTrackEdges divides each edge by the seed's best
  score in its whole cached answer. The cache keeps the raw answer.
- The worker stores matched and unmatched artist rows over the seed's
  top artist.
- Migration 0096 rescales the rows already stored.
- The candidate arms hold sim_score bands so tier 1 stays above tier 2:
  lb_similar 0.5 + 0.5 x edge, tag_overlap halved, likes_overlap 0.3.
  Otherwise a seed's weaker LB edges fell below a shared genre tag.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/similarity/worker.go

@@ -199,6 +199,10 @@ func (w *Worker) upsertArtistSimilar(ctx context.Context, q *dbq.Queries, artist
 		return
 	}
 	sort.Slice(results, func(i, j int) bool { return results[i].Score > results[j].Score })
+	// Scores are stored relative to the seed's closest artist, as track edges
+	// are (#6177): LB's raw scores are counts (16 to 10,875 on the operator's
+	// library), and both the candidate pool and the suggestions read 0 to 1.
+	best := results[0].Score
 
 	mbids := make([]string, 0, len(results))
 	for _, r := range results {
@@ -230,7 +234,7 @@ func (w *Worker) upsertArtistSimilar(ctx context.Context, q *dbq.Queries, artist
 			continue
 		}
 		if uerr := q.UpsertArtistSimilarity(ctx, dbq.UpsertArtistSimilarityParams{
-			ArtistAID: artistAID, ArtistBID: localID, Score: r.Score,
+			ArtistAID: artistAID, ArtistBID: localID, Score: relativeScore(r.Score, best),
 		}); uerr != nil {
 			w.logger.Warn("similarity: UpsertArtistSimilarity", "err", uerr)
 			continue
@@ -257,7 +261,7 @@ func (w *Worker) upsertArtistSimilar(ctx context.Context, q *dbq.Queries, artist
 			SeedArtistID:  artistAID,
 			CandidateMbid: r.MBID,
 			CandidateName: r.Name,
-			Score:         r.Score,
+			Score:         relativeScore(r.Score, best),
 			Source:        "listenbrainz",
 		}); uerr != nil {
 			w.logger.Warn("similarity: UpsertArtistSimilarityUnmatched", "err", uerr)
@@ -266,3 +270,12 @@ func (w *Worker) upsertArtistSimilar(ctx context.Context, q *dbq.Queries, artist
 		takenUnmatched++
 	}
 }
+
+// relativeScore is score over the seed's best, in [0, 1]; 0 when the seed has
+// no positive score to measure against.
+func relativeScore(score, best float64) float64 {
+	if best <= 0 {
+		return 0
+	}
+	return score / best
+}