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/resources/mapper/KeyboardCharacter‌Mapper.xml

@@ -4,7 +4,7 @@
   <resultMap id="BaseResultMap" type="com.yolo.keyborad.model.entity.KeyboardCharacter">
     <!--@Table keyboard_character-->
     <id column="id" jdbcType="BIGINT" property="id" />
-    <result column="title" jdbcType="VARCHAR" property="title" />
+    <result column="character_name" jdbcType="VARCHAR" property="characterName" />
     <result column="character_background" jdbcType="VARCHAR" property="characterBackground" />
     <result column="avatar_url" jdbcType="VARCHAR" property="avatarUrl" />
     <result column="download" jdbcType="VARCHAR" property="download" />
@@ -16,7 +16,7 @@
     <result column="rank" jdbcType="INTEGER" property="rank" />
   </resultMap>
   <sql id="Base_Column_List">
-    id, title, "character_background", avatar_url, download, tag, deleted, created_at,
+    id, character_name, "character_background", avatar_url, download, tag, deleted, created_at,
     updated_at, prompt, "rank"
   </sql>
 </mapper>