fix: use 100K reference floor for popularity normalization

When maxPop=0 (no artist has index/LB popularity data), blendedScore
returned raw relevance (0-1), making Score = MB_score directly.
Shannon Hale (MB 83, zero listens) scored 92 after tier adjustment
and ranked #4 — above Shannon Wright (MB 80, 766K real listens but
not in index).

Now blendedScore uses max(maxPop, 100K) as the normalization
denominator. With zero popularity against a 100K reference, the
60% popularity component contributes near-zero, dropping all
zero-pop artists to ~35-40. This ensures unpopular artists can't
dominate through MB text relevance alone when the index lacks data.

backend/explore/explore.go

@@ -1372,11 +1372,15 @@ func rerankReleaseGroups(rgs []MBReleaseGroup, pop map[string]int) {
 // relevance is 0–1. listenCount is raw; maxListenCount is the
 // maximum in the result set (for normalization).
 func blendedScore(relevance float64, listenCount, maxListenCount int) float64 {
-	if maxListenCount <= 0 {
-		return relevance
+	// Use a floor for maxListenCount so that zero-popularity artists
+	// don't get a free pass when no result has popularity data.
+	// 100K is a reasonable "average popular artist" reference point.
+	effectiveMax := maxListenCount
+	if effectiveMax < 100_000 { //nolint:mnd
+		effectiveMax = 100_000
 	}
 
-	logPop := math.Log10(float64(listenCount)+1) / math.Log10(float64(maxListenCount)+1)
+	logPop := math.Log10(float64(listenCount)+1) / math.Log10(float64(effectiveMax)+1)
 
 	return relevanceWeight*relevance + popularityWeight*logPop
 }