add sqllite utils

lib/models/message_model.dart

@@ -0,0 +1,13 @@
+class Message {
+  final String role;  // 'user' 或 'assistant'
+  final String message;
+
+  Message({required this.role, required this.message});
+
+  Map<String, dynamic> toMap() {
+    return {
+      'role': role,
+      'message': message,
+    };
+  }
+}