Optimize GetArtists: parallel artist processing, batch album upserts, batch download lookups, retry on metadata calls

internal/database/download_repository.go

@@ -153,6 +153,31 @@ func (r *DownloadRepository) GetByID(ctx context.Context, id string) (*Download,
 	return d, nil
 }
 
+func (r *DownloadRepository) GetLatestByAlbumIDs(ctx context.Context, albumIDs []string) (map[string]*Download, error) {
+	if len(albumIDs) == 0 {
+		return nil, nil
+	}
+
+	rows, err := r.pool.Query(ctx,
+		`SELECT DISTINCT ON (album_id) id, torrent_id, album_id, format, quality, state, qbit_hash, save_path, error_message, queued_at, started_at, completed_at, created_at, updated_at
+		 FROM downloads WHERE album_id = ANY($1) ORDER BY album_id, created_at DESC`, albumIDs,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("batch listing downloads: %w", err)
+	}
+	defer rows.Close()
+
+	result := make(map[string]*Download, len(albumIDs))
+	for rows.Next() {
+		d := &Download{}
+		if err := rows.Scan(&d.ID, &d.TorrentID, &d.AlbumID, &d.Format, &d.Quality, &d.State, &d.QbitHash, &d.SavePath, &d.ErrorMessage, &d.QueuedAt, &d.StartedAt, &d.CompletedAt, &d.CreatedAt, &d.UpdatedAt); err != nil {
+			return nil, fmt.Errorf("scanning download: %w", err)
+		}
+		result[d.AlbumID] = d
+	}
+	return result, nil
+}
+
 func (r *DownloadRepository) HasAlbumInQuality(ctx context.Context, albumID string, format string, quality string) (bool, error) {
 	var exists bool
 	err := r.pool.QueryRow(ctx,