feat(character): 新增删除用户人设接口

新增 /character/delUserCharacter 接口，仅允许删除当前用户的人设，并在错误码中补充删除失败场景。

src/main/resources/mapper/KeyboardUserCharacterMapper.xml

@@ -33,4 +33,10 @@
         FROM keyboard_user_sort
         WHERE user_id = #{userId}
     </select>
+
+    <delete id="deleteByIdAndUserId">
+        DELETE FROM keyboard_user_character
+        WHERE id = #{id}
+        AND user_id = #{userId}
+    </delete>
 </mapper>