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/Application/Response/ChapterResponse.php

@@ -11,6 +11,6 @@ readonly class ChapterResponse
         public ?int $volume,
         public bool $isVisible,
         public ?string $pagesDirectory,
-        public \DateTimeImmutable $createdAt
+        public string $createdAt
     ) {}
 }