feat(character): 新增用户人设列表接口并支持邮箱登录

- CharacterController 增加 /listByUser 端点，返回当前用户已购人设
- KeyboardCharacterService 新增 selectListByUserId()，通过 Sa-Token 取当前用户 ID
- 引入 KeyboardUserCharacter 中间表及对应 Mapper、VO
- UserController 增加 /login 端点，支持邮箱+密码登录
- 统一将实体与 VO 的 title 字段更名为 characterName
- 补充错误码 USER_NOT_FOUND，调整 Sa-Token 白名单与 Redis 依赖

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

@@ -0,0 +1,12 @@
+package com.yolo.keyborad.mapper;
+
+import com.baomidou.mybatisplus.core.mapper.BaseMapper;
+import com.yolo.keyborad.model.entity.KeyboardUserCharacter;
+
+/*
+* @author: ziin
+* @date: 2025/12/3 15:43
+*/
+    
+public interface KeyboardUserCharacterMapper extends BaseMapper<KeyboardUserCharacter> {
+}