feat: artist aliases in FTS5 index + BM25 blended scoring

Migration 14: add aliases TEXT column to explore_index, rebuild FTS5
with 3 columns (title, artist_name, aliases), recreate sync triggers.
Clears index build timestamps to force alias population on next build.

Artist image provider fetches inc=url-rels+aliases (single call, no
extra cost). GetAliases() extracts alias names from cached MB rels.
indexOneArtist stores aliases as space-separated text after image
resolution populates the cache.

Search query now uses BM25 blended scoring:
  ORDER BY bm25(fts, 3.0, 1.0, 0.5) - (ln(popularity+1) * 0.5)

Column weights: title=3.0, artist_name=1.0, aliases=0.5
- Title matches score 3x higher than artist name matches
- Alias matches are helpful but don't dominate
- Popularity is a log-scaled boost, not an override
- Exact title match on niche entity beats weak match on mega-popular

Enables: 'rhcp' → Red Hot Chili Peppers, 'gnr' → Guns N' Roses,
'sabbath' → Black Sabbath (once index build runs with aliases).

backend/explore/artistimage.go

@@ -168,7 +168,7 @@ func (p *ArtistImageProvider) fetchMBRels(artistMBID string) []mbRelation {
 	}
 
 	url := fmt.Sprintf(
-		"https://musicbrainz.org/ws/2/artist/%s?fmt=json&inc=url-rels",
+		"https://musicbrainz.org/ws/2/artist/%s?fmt=json&inc=url-rels+aliases",
 		artistMBID,
 	)
 
@@ -368,6 +368,38 @@ func wikimediaThumbURL(filename string) string {
 	)
 }
 
+// GetAliases returns the artist's aliases as a space-separated
+// string, extracted from the cached MB rels response.  Returns ""
+// if no aliases are cached.
+func (p *ArtistImageProvider) GetAliases(artistMBID string) string {
+	cacheKey := "mb:artist-rels:" + artistMBID
+
+	data, ok := p.cache.Get(cacheKey)
+	if !ok {
+		return ""
+	}
+
+	var envelope struct {
+		Aliases []struct {
+			Name string `json:"name"`
+		} `json:"aliases"`
+	}
+
+	if err := json.Unmarshal(data, &envelope); err != nil || len(envelope.Aliases) == 0 {
+		return ""
+	}
+
+	names := make([]string, 0, len(envelope.Aliases))
+
+	for _, a := range envelope.Aliases {
+		if a.Name != "" {
+			names = append(names, a.Name)
+		}
+	}
+
+	return strings.Join(names, " ")
+}
+
 func (p *ArtistImageProvider) fetchURL(url string) ([]byte, error) {
 	ctx, cancel := context.WithTimeout(context.Background(), artistImageTimeout)
 	defer cancel()

backend/explore/searchindex.go

@@ -82,6 +82,7 @@ type SearchIndexResult struct {
 	ArtistMBID string `json:"artistMbid"`
 	Popularity int    `json:"popularity"`
 	ExtraJSON  string `json:"extraJson,omitempty"`
+	Aliases    string `json:"aliases,omitempty"`
 }
 
 // lbSitewideArtist is the response shape from the LB sitewide
@@ -253,7 +254,7 @@ 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 i.popularity DESC
+		ORDER BY bm25(explore_index_fts, 3.0, 1.0, 0.5) - (ln(i.popularity + 1) * 0.5)
 		LIMIT ?
 	`, ftsQuery, limit)
 	if err != nil {
@@ -1014,6 +1015,23 @@ func (si *SearchIndex) indexOneArtist(
 
 	wg.Wait()
 
+	// Extract aliases from the now-cached MB rels (populated by
+	// the image resolution above) and update the artist's index entry.
+	if si.artistImg != nil {
+		aliases := si.artistImg.GetAliases(artist.ArtistMBID)
+		if aliases != "" {
+			si.writeBatch([]SearchIndexResult{{
+				EntityType: "artist",
+				MBID:       artist.ArtistMBID,
+				Title:      artist.ArtistName,
+				ArtistName: artist.ArtistName,
+				ArtistMBID: artist.ArtistMBID,
+				Popularity: artist.ListenCount,
+				Aliases:    aliases,
+			}})
+		}
+	}
+
 	// Batch write discography results.
 	all := make([]SearchIndexResult, 0, len(rgs)+len(recs))
 	all = append(all, rgs...)
@@ -1216,9 +1234,9 @@ func (si *SearchIndex) writeBatch(entries []SearchIndexResult) {
 	for _, e := range entries {
 		if _, err := tx.Exec(`
 			INSERT OR REPLACE INTO explore_index
-				(entity_type, mbid, title, artist_name, artist_mbid, popularity, extra_json)
-			VALUES (?, ?, ?, ?, ?, ?, NULLIF(?, ''))
-		`, e.EntityType, e.MBID, e.Title, e.ArtistName, e.ArtistMBID, e.Popularity, e.ExtraJSON,
+				(entity_type, mbid, title, artist_name, artist_mbid, popularity, extra_json, aliases)
+			VALUES (?, ?, ?, ?, ?, ?, NULLIF(?, ''), NULLIF(?, ''))
+		`, e.EntityType, e.MBID, e.Title, e.ArtistName, e.ArtistMBID, e.Popularity, e.ExtraJSON, e.Aliases,
 		); err != nil {
 			si.logger.Warn("search index: insert error",
 				"mbid", e.MBID,