feat(character): 添加用户人设接口及排序管理

新增 /character/addUserCharacter 端点，支持用户将人设加入个人列表并自动维护排序数组。引入 KeyboardUserCharacterAddDTO、KeyboardUserSortMapper 及相关错误码，实现事务级插入与排序更新。

src/main/resources/mapper/KeyboardUserSortMapper.xml

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
+<mapper namespace="com.yolo.keyborad.mapper.KeyboardUserSortMapper">
+    <!-- 展开数组为多行：返回 List<Long> -->
+    <select id="selectSortByUserId" resultType="java.lang.Long">
+        SELECT unnest(user_characteu_id_sort)
+        FROM keyboard_user_sort
+        WHERE user_id = #{userId}
+    </select>
+
+    <update id="updateSortByUserId">
+        UPDATE keyboard_user_sort
+        SET user_characteu_id_sort = #{sort, jdbcType=ARRAY}
+        WHERE user_id = #{userId}
+    </update>
+
+    <insert id="insertUserSort">
+        INSERT INTO keyboard_user_sort (user_id, user_characteu_id_sort)
+        VALUES (#{userId}, #{sort, jdbcType=ARRAY})
+    </insert>
+
+</mapper>