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

bin/models/session.dart

@@ -0,0 +1,37 @@
+class Session {
+  final String id;
+  final String userId;
+  final String token;
+  final DateTime expiresAt;
+  final DateTime createdAt;
+
+  Session({
+    required this.id,
+    required this.userId,
+    required this.token,
+    required this.expiresAt,
+    required this.createdAt,
+  });
+
+  factory Session.fromMap(Map<String, dynamic> map) {
+    return Session(
+      id: map['id'] as String,
+      userId: map['user_id'] as String,
+      token: map['token'] as String,
+      expiresAt: DateTime.parse(map['expires_at'] as String),
+      createdAt: DateTime.parse(map['created_at'] as String),
+    );
+  }
+
+  Map<String, dynamic> toMap() {
+    return {
+      'id': id,
+      'user_id': userId,
+      'token': token,
+      'expires_at': expiresAt.toIso8601String(),
+      'created_at': createdAt.toIso8601String(),
+    };
+  }
+
+  bool get isExpired => DateTime.now().isAfter(expiresAt);
+}