Refactor ExportGeneration to improve serialization and usage

Streamlined the `ExportGeneration` entity by adding a `getStatus` accessor and updating the serialization group. Refactored frontend logic to use computed properties for `status` and `storedObject`. Simplified API methods to work with the updated `ExportGeneration` interface for consistent handling.

src/Bundle/ChillMainBundle/Controller/ExportGenerationController.php

@@ -59,7 +59,7 @@ final readonly class ExportGenerationController
 
         return new JsonResponse(
             $this->serializer->serialize(
-                ['status' => $exportGeneration->getStoredObject()->getStatus(), 'stored_object' => $exportGeneration->getStoredObject()],
+                $exportGeneration,
                 'json',
                 [AbstractNormalizer::GROUPS => ['read']],
             ),