feat: rerank MB results with index popularity, increase popularity weight

Two changes:

1. Rerank MB results using index popularity (no API calls):
   When the index is ready, boostWithIndexPopularity looks up each
   MB result's MBID in the local index to get cached listen counts,
   then reranks using the same blended score formula. This was
   previously skipped entirely for speed, leaving MB results sorted
   by text relevance only — obscure exact matches beat popular
   partial matches.

2. Increase popularity weight across both scoring systems:
   - Blended score: 60% popularity / 40% relevance (was 40/60)
   - FTS5 index: ln(pop+1) * 1.5 factor (was 0.5)

   Result: 'flatbush' → Flatbush Zombies (97) beats 'Flatbush'
   nobody (61). Popular artists with partial name matches now
   reliably outrank obscure exact matches.

backend/explore/searchindex.go

@@ -190,6 +190,33 @@ func (si *SearchIndex) IsReady() bool {
 	return si.ready
 }
 
+// GetPopularity returns the cached popularity (listen count) for
+// the given MBID from the local index.  Returns 0 if not found.
+func (si *SearchIndex) GetPopularity(mbid string) int {
+	if mbid == "" {
+		return 0
+	}
+
+	rows, err := si.db.QueryContext(
+		"SELECT popularity FROM explore_index WHERE mbid = ? LIMIT 1",
+		mbid,
+	)
+	if err != nil {
+		return 0
+	}
+
+	defer func() { _ = rows.Close() }()
+
+	if rows.Next() {
+		var pop int
+		if err := rows.Scan(&pop); err == nil {
+			return pop
+		}
+	}
+
+	return 0
+}
+
 // AddFromCache inserts entries from a cached discography browse
 // into the search index (Tier 5: organic growth).  Called when a
 // user views an artist page and the discography is fetched.
@@ -254,7 +281,7 @@ func (si *SearchIndex) Search(query string, limit int) []SearchIndexResult {
 		FROM explore_index i
 		JOIN explore_index_fts f ON f.rowid = i.id
 		WHERE explore_index_fts MATCH ?
-		ORDER BY bm25(explore_index_fts, 3.0, 1.0, 0.5) - (ln(i.popularity + 1) * 0.5)
+		ORDER BY bm25(explore_index_fts, 3.0, 1.0, 0.5) - (ln(i.popularity + 1) * 1.5)
 		LIMIT ?
 	`, ftsQuery, limit)
 	if err != nil {