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()