fix: distinguish unknown popularity from confirmed zero

Artists not in the explore index had HasPopularity=false and
Popularity=0, making them indistinguishable from confirmed
zero-popularity artists like Shannon Hale. The strict threshold
(60) was filtering all non-indexed MB results.

Now three states:
- Known popular (HasPop=true, Pop>0) → sliding threshold
- Known unpopular (HasPop=true, Pop=0) → strict threshold (60)
- Unknown (HasPop=false) → lenient threshold (15)

Non-indexed MB results are 'unknown' and pass with any reasonable
score. Only artists confirmed to have zero listens face the high bar.

backend/explore/explore.go

@@ -842,12 +842,18 @@ func scalePopularity(listens int) int {
 //	100K listens  → threshold 24
 //	1M+ listens   → threshold 15  (almost anything passes)
 func minScoreForArtist(a MBArtist) int {
+	// Unknown popularity (not in index, no LB lookup yet) —
+	// use lenient threshold since we can't judge.
+	if !a.HasPopularity {
+		return minBlendedScore
+	}
+
+	// Known zero popularity — strict threshold.
 	if a.Popularity <= 0 {
 		return minScoreZeroPop
 	}
 
-	// log10(pop) ranges from ~2 (100 listens) to ~6+ (1M+).
-	// Scale to 0-1 range using 6.0 as the reference ceiling.
+	// Known popularity — threshold slides down with listen count.
 	const logCeiling = 6.0 // log10(1,000,000)
 
 	logPop := math.Log10(float64(a.Popularity))