feat(chat): 新增AI聊天记录持久化功能

新增KeyboardAiChatMessage实体及对应Mapper、Service，在ChatServiceImpl中同步对话时保存用户与AI消息到数据库，实现聊天记录持久化

src/main/java/com/yolo/keyborad/mapper/KeyboardAiChatMessageMapper.java

@@ -0,0 +1,12 @@
+package com.yolo.keyborad.mapper;
+
+import com.baomidou.mybatisplus.core.mapper.BaseMapper;
+import com.yolo.keyborad.model.entity.KeyboardAiChatMessage;
+
+/*
+* @author: ziin
+* @date: 2026/1/26 17:00
+*/
+    
+public interface KeyboardAiChatMessageMapper extends BaseMapper<KeyboardAiChatMessage> {
+}