Added createdAt attribute in dto classes and json schema

lib/data/dto/group.dart

@@ -4,9 +4,9 @@ import 'package:uuid/uuid.dart';
 
 class Group {
   final String id;
+  final DateTime createdAt;
   final String name;
   final List<Player> members;
-  final DateTime createdAt;
 
   Group({
     String? id,
@@ -24,6 +24,7 @@ class Group {
   /// Creates a Group instance from a JSON object.
   Group.fromJson(Map<String, dynamic> json)
     : id = json['id'],
+      createdAt = DateTime.parse(json['createdAt']),
       name = json['name'],
       members = (json['members'] as List)
           .map((memberJson) => Player.fromJson(memberJson))
@@ -32,6 +33,7 @@ class Group {
   /// Converts the Group instance to a JSON object.
   Map<String, dynamic> toJson() => {
     'id': id,
+    'createdAt': createdAt.toIso8601String(),
     'name': name,
     'members': members.map((member) => member.toJson()).toList(),
   };