feat(keyboard): 新增键盘用户管理模块

新增 keyboard-server 子模块，含用户 DO/Mapper/Service/Controller 全套代码；
补充错误码 KEYBOARD_USER_NOT_EXISTS；
主工程引入 keyboard-server 依赖并扩展包扫描路径。

yolo-server/src/main/java/com/yolo/keyboard/server/YoloServerApplication.java

@@ -13,7 +13,7 @@ import org.springframework.boot.autoconfigure.SpringBootApplication;
  * @author 芋道源码
  */
 @SuppressWarnings("SpringComponentScan") // 忽略 IDEA 无法识别 ${yolo.info.base-package}
-@SpringBootApplication(scanBasePackages = {"${yolo.info.base-package}.server", "${yolo.info.base-package}.module"})
+@SpringBootApplication(scanBasePackages = {"${yolo.info.base-package}.server", "${yolo.info.base-package}.module", "com.yolo.keyboard"})
 public class YoloServerApplication {
 
     public static void main(String[] args) {