fix : correctifs de sécurité et robustesse post-review

- MeProvider : guard null user avec AccessDeniedHttpException
- MaintenanceToggleProcessor : vérification des opérations filesystem
- User : restreindre Get/GetCollection aux ROLE_ADMIN
- useAppVersion : corriger le path relatif '/version'

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Entity/User.php

@@ -28,9 +28,11 @@ use Symfony\Component\Serializer\Attribute\Groups;
             normalizationContext: ['groups' => ['me:read']],
         ),
         new Get(
+            security: "is_granted('ROLE_ADMIN')",
             normalizationContext: ['groups' => ['user:list']],
         ),
         new GetCollection(
+            security: "is_granted('ROLE_ADMIN')",
             normalizationContext: ['groups' => ['user:list']],
         ),
         new Post(security: "is_granted('ROLE_ADMIN')", processor: UserPasswordHasherProcessor::class),