fix: raise minBlendedScore from 25 to 50 to filter zero-popularity artists

Shannon Hale had zero LB listens but survived filtering with a score
of 37 (from MB text relevance alone). At minBlendedScore=50, artists
with no listening data and only partial name matches are filtered out.
Every artist with actual LB popularity data still passes the threshold.

backend/explore/explore.go

@@ -898,8 +898,10 @@ const (
 	maxResults = 15
 
 	// minBlendedScore is the floor for artists and recordings
-	// after popularity reranking (0–100 scale).
-	minBlendedScore = 25
+	// after popularity reranking and tier adjustment (0–100 scale).
+	// At 50, artists with zero LB popularity and partial name
+	// matches are filtered out.
+	minBlendedScore = 50
 )
 
 // tierBonus maps artist name-match tiers to percentage score multipliers.