fix(bundle/exporter): add rows.Err() after child workspace enumeration

Silent data loss on mid-cursor DB errors — partial sub-workspace
bundles returned instead of surfacing the iteration error. Adds
rows.Err() check after the SELECT id FROM workspaces query in
Export(), mirroring the pattern already used in scheduler.go
and handlers with similar recursion patterns.

Closes: R1 MISSING-ROWS-ERR findings (bundle/exporter.go)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

workspace-server/internal/bundle/exporter.go

@@ -92,6 +92,9 @@ func Export(ctx context.Context, workspaceID, configsDir string, dockerCli *clie
 				}
 			}
 		}
+		if err := rows.Err(); err != nil {
+			return nil, fmt.Errorf("export sub-workspaces: %w", err)
+		}
 	}
 
 	return b, nil