feat: Add initial backend server with user authentication, session management, and playlist CRUD functionality.

bin/models/user.dart

@@ -0,0 +1,31 @@
+class User {
+  final String id;
+  final String username;
+  final bool isAdmin;
+  final DateTime createdAt;
+
+  User({
+    required this.id,
+    required this.username,
+    required this.isAdmin,
+    required this.createdAt,
+  });
+
+  factory User.fromMap(Map<String, dynamic> map) {
+    return User(
+      id: map['id'] as String,
+      username: map['username'] as String,
+      isAdmin: (map['is_admin'] as int) == 1,
+      createdAt: DateTime.parse(map['created_at'] as String),
+    );
+  }
+
+  Map<String, dynamic> toJson() {
+    return {
+      'id': id,
+      'username': username,
+      'isAdmin': isAdmin,
+      'createdAt': createdAt.toIso8601String(),
+    };
+  }
+}