fix: backfill updates only missing artists, preserves index scores

The previous backfill called rerankArtists with an incomplete pop
map (only backfilled artists), wiping out scores for artists that
had index data (including the library-boosted Shannon and the Clams).

Now backfill only updates Score for artists that were actually
backfilled from LB, using OriginalScore as the relevance input
and a maxPop computed across both index and backfill data. Artists
with existing index scores are untouched. A final sort by Score
merges both groups into the correct order.

backend/explore/explore.go

@@ -1072,35 +1072,40 @@ func (e *Service) backfillArtistPopularity(result *MBSearchResult) {
 		return
 	}
 
-	// Build library set for the re-sort.
-	libCheck := e.libMBID.CheckMBIDs(missing)
-	libMBIDs := make(map[string]bool)
+	// Find the maxPop across ALL artists (both index and backfilled)
+	// so normalization is consistent.
+	maxPop := 0
 
-	for mbid, entityType := range libCheck {
-		if entityType == "artist" {
-			libMBIDs[mbid] = true
+	for _, a := range result.Artists {
+		if a.Popularity > maxPop {
+			maxPop = a.Popularity
 		}
 	}
 
-	// Merge into a combined pop map (index + backfill).
+	for _, p := range pop {
+		if p > maxPop {
+			maxPop = p
+		}
+	}
+
+	// Update scores only for artists that got backfilled.
+	// Preserve scores of artists that already had index data.
 	for i := range result.Artists {
 		a := &result.Artists[i]
 		if p, ok := pop[a.MBID]; ok {
 			a.HasPopularity = true
 			a.Popularity = p
+
+			rel := float64(a.OriginalScore) / 100.0
+			s := blendedScore(rel, p, maxPop)
+			a.Score = int(s * 100)
 		}
 	}
 
-	// Re-derive scores with the new popularity data.
-	allPop := make(map[string]int, len(result.Artists))
-
-	for _, a := range result.Artists {
-		if a.Popularity > 0 {
-			allPop[a.MBID] = a.Popularity
-		}
-	}
-
-	rerankArtists(result.Artists, allPop, libMBIDs)
+	// Re-sort all artists by score (index-scored + backfill-scored).
+	sort.SliceStable(result.Artists, func(i, j int) bool {
+		return result.Artists[i].Score > result.Artists[j].Score
+	})
 }
 
 func (e *Service) boostWithPopularity(result *MBSearchResult) {