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/recsettings/service.go

@@ -136,7 +136,7 @@ func ShippedDailyMixWeights() recommendation.ScoringWeights {
 // admin tuning card and good values get baked back here. They are a
 // defensible starting point rather than a measured optimum: the per-arm fill
 // rates and the real sim_score distribution are still unknown (#3879), and
-// `likes_overlap` contributes a flat 0.6 that a high SimilarityWeight
+// `likes_overlap` contributes a flat 0.3 that a high SimilarityWeight
 // amplifies. Expect to move these once that lands.
 func ShippedSongsLikeWeights() recommendation.ScoringWeights {
 	return recommendation.ScoringWeights{