perf: fix N+1 query in GetAllByArtistID with batch loading

Replace per-album loadRelations calls with batch queries using ANY($1).
Reduces 1+2N queries to 3 queries total (1.45s → 50ms for 83 albums).

internal/repository/postgres/album.go

@@ -122,18 +122,94 @@ func (r *AlbumRepository) GetAllByArtistID(ctx context.Context, artistID string)
 	defer rows.Close()
 
 	var albums []domain.Album
+	var albumIDs []string
+	albumIndex := make(map[string]int)
+
 	for rows.Next() {
 		album, err := r.scanAlbumFromRow(rows)
 		if err != nil {
 			return nil, err
 		}
-		if err := r.loadRelations(ctx, album); err != nil {
-			return nil, err
-		}
+		albumIndex[album.ID] = len(albums)
+		albumIDs = append(albumIDs, album.ID)
 		albums = append(albums, *album)
 	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
 
-	return albums, rows.Err()
+	if len(albums) == 0 {
+		return albums, nil
+	}
+
+	if err := r.loadRelationsBatch(ctx, albums, albumIDs, albumIndex); err != nil {
+		return nil, err
+	}
+
+	return albums, nil
+}
+
+func (r *AlbumRepository) loadRelationsBatch(ctx context.Context, albums []domain.Album, albumIDs []string, albumIndex map[string]int) error {
+	extQuery := `SELECT album_id, source, source_id, url FROM album_external_ids WHERE album_id = ANY($1)`
+	extRows, err := r.pool.Query(ctx, extQuery, albumIDs)
+	if err != nil {
+		return err
+	}
+	defer extRows.Close()
+
+	for extRows.Next() {
+		var albumID string
+		var ext domain.ExternalID
+		var url *string
+		if err := extRows.Scan(&albumID, &ext.Source, &ext.SourceID, &url); err != nil {
+			return err
+		}
+		ext.URL = derefString(url)
+		if idx, ok := albumIndex[albumID]; ok {
+			albums[idx].ExternalIDs = append(albums[idx].ExternalIDs, ext)
+		}
+	}
+	if err := extRows.Err(); err != nil {
+		return err
+	}
+
+	artistQuery := `
+		SELECT aa.album_id, a.id, a.name, a.sort_name, a.artist_type, a.country,
+		       aa.role, aa.position
+		FROM album_artists aa
+		JOIN artists a ON a.id = aa.artist_id
+		WHERE aa.album_id = ANY($1)
+		ORDER BY aa.album_id, aa.position`
+
+	artistRows, err := r.pool.Query(ctx, artistQuery, albumIDs)
+	if err != nil {
+		return err
+	}
+	defer artistRows.Close()
+
+	for artistRows.Next() {
+		var (
+			albumID  string
+			ac       domain.ArtistCredit
+			sortName *string
+			artType  *string
+			country  *string
+		)
+		if err := artistRows.Scan(
+			&albumID, &ac.Artist.ID, &ac.Artist.Name, &sortName, &artType, &country,
+			&ac.Role, &ac.Position,
+		); err != nil {
+			return err
+		}
+		ac.Artist.SortName = derefString(sortName)
+		ac.Artist.Type = derefString(artType)
+		ac.Artist.Country = derefString(country)
+		if idx, ok := albumIndex[albumID]; ok {
+			albums[idx].Artists = append(albums[idx].Artists, ac)
+		}
+	}
+
+	return artistRows.Err()
 }
 
 func (r *AlbumRepository) SaveAll(ctx context.Context, albums []domain.Album) error {