fix: answer null instead of 500 when the export marker is missing

src/Controller/Api/LastUpdateController.php

@@ -2,6 +2,7 @@
 
 namespace App\Controller\Api;
 
+use App\BusProNet\Model\XmlExportInfo;
 use League\Flysystem\FilesystemOperator;
 use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
 use Symfony\Component\HttpFoundation\JsonResponse;
@@ -16,23 +17,15 @@ class LastUpdateController extends AbstractController
     {
     }
 
+    /**
+     * A missing or unreadable marker answers `{"timestamp": null}`, never a 500: consumers
+     * poll this for change detection and treat null as "unknown".
+     */
     #[Route(path: '/last-update', name: 'api_last_update', methods: ['GET'])]
     public function index(): JsonResponse
     {
-        if (false === $this->xmlExport->has('uebertragung.info')) {
-            $lastUploadAt = null;
-        } else {
-            $file = $this->xmlExport->read('uebertragung.info');
-            $lines = explode("\n", $file);
-            $line = trim($lines[0]);
+        $info = XmlExportInfo::fromStorage($this->xmlExport);
 
-            if (true === empty($line)) {
-                $lastUploadAt = null;
-            } else {
-                $lastUploadAt = \DateTime::createFromFormat('d.m.Y H:i:s', $line);
-            }
-        }
-
-        return $this->json(['timestamp' => $lastUploadAt->format('Y-m-d H:i:s')]);
+        return $this->json(['timestamp' => $info?->lastTransfer->format('Y-m-d H:i:s')]);
     }
 }