feat: report the source export timestamp on every travel export page

src/Controller/Api/ExportController.php

@@ -59,6 +59,7 @@ class ExportController extends AbstractController
                 'totalPages' => (int) ceil($total / $query->perPage),
                 'skipped' => $export->skipped,
                 'pickupsWithoutMasterData' => $export->pickupsWithoutMasterData,
+                'sourceGeneratedAt' => $export->sourceGeneratedAt?->format('c'),
                 'generatedAt' => $export->generatedAt->format('c'),
             ],
         ]);