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_integration_test.go

@@ -209,6 +209,38 @@ func TestTickOnce_MapsLBResponseToLocalLibrary(t *testing.T) {
 	}
 }
 
+// LB's scores are counts, not 0 to 1 (16 to 1445 on the operator's library).
+// An edge is stored over the seed's best score in the whole answer, in the
+// library or not, so the scorer reads it as 0 to 1 (#6177).
+func TestTickOnce_EdgeIsRelativeToTheSeedsBest(t *testing.T) {
+	f := newFixture(t)
+	mbidA := "11111111-1111-1111-1111-111111111111"
+	mbidB := "22222222-2222-2222-2222-222222222222"
+	mbidC := "99999999-9999-9999-9999-999999999999" // NOT in library
+	trackA := seedTrack(t, f, "A", &mbidA)
+	trackB := seedTrack(t, f, "B", &mbidB)
+	markPlayed(t, f, trackA.ID)
+	body := `[
+		{"recording_mbid": "` + mbidC + `", "score": 1445},
+		{"recording_mbid": "` + mbidB + `", "score": 289}
+	]`
+	srv := stubLB(body, `[]`, http.StatusOK)
+	defer srv.Close()
+	w := newTestWorker(f, srv.URL)
+	if err := w.tickOnce(context.Background()); err != nil {
+		t.Fatalf("tickOnce: %v", err)
+	}
+	var score float64
+	if err := f.pool.QueryRow(context.Background(),
+		`SELECT score FROM track_similarity WHERE track_a_id = $1 AND track_b_id = $2 AND source = 'listenbrainz'`,
+		trackA.ID, trackB.ID).Scan(&score); err != nil {
+		t.Fatalf("edge: %v", err)
+	}
+	if score != 0.2 {
+		t.Errorf("score = %v, want 0.2 (289 over the seed's best, 1445)", score)
+	}
+}
+
 // The 20-per-seed cap is gone with the cache (#5296): LB's answer is at most
 // 50, and every recording of it that is in the library becomes an edge.
 func TestTickOnce_KeepsEveryInLibraryMatch(t *testing.T) {
@@ -279,8 +311,9 @@ func TestTickOnce_RefreshesStaleRow(t *testing.T) {
 	_ = f.pool.QueryRow(context.Background(),
 		`SELECT score, fetched_at FROM track_similarity WHERE track_a_id = $1 AND track_b_id = $2 AND source = 'listenbrainz'`,
 		trackA.ID, other.ID).Scan(&score, &fetchedAt)
-	if score != 0.95 {
-		t.Errorf("score = %v, want 0.95 (refreshed)", score)
+	// The seed's only answer is its best, so the refreshed edge is 1.0 (#6177).
+	if score != 1.0 {
+		t.Errorf("score = %v, want 1.0 (refreshed, relative to the seed's best)", score)
 	}
 	if time.Since(fetchedAt) > time.Minute {
 		t.Errorf("fetched_at not bumped: %v ago", time.Since(fetchedAt))
@@ -419,11 +452,12 @@ func TestUpsertArtistSimilar_PersistsUnmatchedToTable(t *testing.T) {
 	w := newTestWorker(f, "")
 
 	similars := []listenbrainz.SimilarArtist{
-		{MBID: inLibMBID, Name: "InLib Artist", Score: 0.95},
-		{MBID: "out-mbid-1", Name: "Outsider One", Score: 0.85},
-		{MBID: "out-mbid-2", Name: "Outsider Two", Score: 0.80},
-		{MBID: "out-mbid-3", Name: "Outsider Three", Score: 0.70},
-		{MBID: "out-mbid-4", Name: "", Score: 0.60}, // empty name — skipped
+		// LB's scores are counts; rows are stored over the seed's best (#6177).
+		{MBID: inLibMBID, Name: "InLib Artist", Score: 1000},
+		{MBID: "out-mbid-1", Name: "Outsider One", Score: 850},
+		{MBID: "out-mbid-2", Name: "Outsider Two", Score: 800},
+		{MBID: "out-mbid-3", Name: "Outsider Three", Score: 700},
+		{MBID: "out-mbid-4", Name: "", Score: 600}, // empty name — skipped
 	}
 	w.upsertArtistSimilar(ctx, f.q, f.artist.ID, similars)
 
@@ -438,6 +472,16 @@ func TestUpsertArtistSimilar_PersistsUnmatchedToTable(t *testing.T) {
 	if matchedCount != 1 {
 		t.Errorf("artist_similarity rows = %d, want 1 (only the in-library match)", matchedCount)
 	}
+	var matchedScore float64
+	if err := f.pool.QueryRow(ctx,
+		"SELECT score FROM artist_similarity WHERE artist_a_id = $1 AND artist_b_id = $2",
+		f.artist.ID, inLibArtist.ID,
+	).Scan(&matchedScore); err != nil {
+		t.Fatalf("fetch matched score: %v", err)
+	}
+	if matchedScore != 1.0 {
+		t.Errorf("matched score = %v, want 1.0 (the seed's best)", matchedScore)
+	}
 
 	// Unmatched path: 3 rows (out-mbid-1/2/3); the empty-name row is skipped.
 	var unmatchedCount int
@@ -464,7 +508,6 @@ func TestUpsertArtistSimilar_PersistsUnmatchedToTable(t *testing.T) {
 		t.Errorf("row = (%q, %v), want (Outsider One, 0.85)", name, score)
 	}
 
-	_ = inLibArtist
 }
 
 // recordFetch stamps a seed as answered by ListenBrainz at the given SQL time.