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

tests/Controller/Api/ExportControllerTest.php

@@ -30,7 +30,7 @@ class ExportControllerTest extends TestCase
         $first = $this->call(new TravelExportQuery(1, 5))['meta'];
         $last = $this->call(new TravelExportQuery(3, 5))['meta'];
 
-        foreach (['total', 'totalPages', 'skipped', 'pickupsWithoutMasterData', 'generatedAt'] as $key) {
+        foreach (['total', 'totalPages', 'skipped', 'pickupsWithoutMasterData', 'sourceGeneratedAt', 'generatedAt'] as $key) {
             self::assertSame($first[$key], $last[$key], $key.' must not vary by page');
         }
     }
@@ -78,6 +78,18 @@ class ExportControllerTest extends TestCase
         self::assertSame(7, $meta['skipped']);
         self::assertSame(21, $meta['pickupsWithoutMasterData']);
         self::assertSame('2026-09-29T03:20:00+02:00', $meta['generatedAt']);
+        self::assertSame('2026-09-28T23:10:15+02:00', $meta['sourceGeneratedAt']);
+    }
+
+    public function testAnUnknownSourceExportIsNullRatherThanOmitted(): void
+    {
+        $builder = $this->createStub(TravelExportBuilder::class);
+        $builder->method('build')->willReturn(new TravelExport([], 0, 0, new \DateTimeImmutable()));
+
+        $meta = json_decode((string) $this->controller($builder)->travels(new TravelExportQuery())->getContent(), true)['meta'];
+
+        self::assertArrayHasKey('sourceGeneratedAt', $meta);
+        self::assertNull($meta['sourceGeneratedAt']);
     }
 
     /**
@@ -110,6 +122,7 @@ class ExportControllerTest extends TestCase
             $skipped,
             $withoutMasterData,
             new \DateTimeImmutable('2026-09-29 03:20:00', new \DateTimeZone('Europe/Berlin')),
+            new \DateTimeImmutable('2026-09-28 23:10:15', new \DateTimeZone('Europe/Berlin')),
         ));
 
         $response = $this->controller($builder)->travels($query ?? new TravelExportQuery());