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

src/Model/TravelExport.php

@@ -19,12 +19,14 @@ final readonly class TravelExport
      * @param list<array<string, mixed>> $rows                     one row per (travel, hotel) pair
      * @param int                        $skipped                  (travel, hotel) pairs dropped as unusable
      * @param int                        $pickupsWithoutMasterData distinct pickup ids absent from zustiege.xml
+     * @param \DateTimeImmutable|null    $sourceGeneratedAt        the uebertragung.info timestamp of the export read, null if unknown
      */
     public function __construct(
         public array $rows,
         public int $skipped,
         public int $pickupsWithoutMasterData,
         public \DateTimeImmutable $generatedAt,
+        public ?\DateTimeImmutable $sourceGeneratedAt = null,
     ) {
     }
 }