fix: penalize substring matches (tier 2) with -5% multiplier

'Del Shannon' was ranking above 'Shannon and the Clams' because
tier 2 (substring) had a neutral ×1.0 multiplier. Del Shannon's
MB score of 100 (Lucene considers 'Shannon' a full word match)
plus 588K listens gave him a base score of 98 — nearly untouchable.

Tier 2 now gets -5%, dropping Del Shannon to 93 while starts-with
matches like Shannon Wright (99) and Shannon and the Clams (90)
maintain their advantage. The logic: when the user types 'shannon',
results where 'shannon' starts the name are more likely what they
want than results where it's buried in the middle.

backend/explore/explore.go

@@ -911,7 +911,7 @@ const (
 var tierBonus = map[int]float64{
 	0: 0.15,  // exact match: +15%
 	1: 0.08,  // starts with: +8%
-	2: 0.0,   // substring: no change
+	2: -0.05, // substring (query buried in name): -5%
 	3: -0.15, // no substring match: -15%
 }