创建和更新用户名校验增加校验，不允许纯数字用户名

backend/src/main/java/com/openisle/service/UsernameValidator.java

@@ -1,6 +1,7 @@
 package com.openisle.service;
 
 import com.openisle.exception.FieldException;
+import org.apache.commons.lang3.math.NumberUtils;
 import org.springframework.stereotype.Service;
 
 /**
@@ -17,6 +18,11 @@ public class UsernameValidator {
         if (username == null || username.isEmpty()) {
             throw new FieldException("username", "Username cannot be empty");
         }
+
+        if (NumberUtils.isDigits(username)) {
+            throw new FieldException("username", "Username cannot be pure number");
+        }
     }
+
 }