fix(manga): ChapterResponse.createdAt en string RFC3339

- ChapterResponse expose createdAt comme string formatée (RFC3339)
- GetMangaChaptersHandler formate la date à la construction du DTO
- GetMangaChaptersStateProvider adapté en conséquence

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/Domain/Manga/Infrastructure/ApiPlatform/State/Provider/GetMangaChaptersStateProvider.php

@@ -53,7 +53,7 @@ readonly class GetMangaChaptersStateProvider implements ProviderInterface
             volume: $chapter->volume,
             isVisible: $chapter->isVisible,
             isAvailable: $chapter->pagesDirectory !== null,
-            createdAt: $chapter->createdAt->format(\DateTimeInterface::RFC3339)
+            createdAt: $chapter->createdAt
         );
     }
 }