update. 完善用户相关增删改查接口以及Schema模型

users/models.py

@@ -33,9 +33,22 @@ class Department(BaseEntity):
         verbose_name = "部门"
         verbose_name_plural = "部门管理"
 
+        # 👇 核心：添加部分唯一约束
+        constraints = [
+            models.UniqueConstraint(
+                fields=['parent'],
+                condition=models.Q(parent__isnull=True),
+                name='unique_root_department'
+            )
+        ]
+
     def __str__(self):
         return self.name
 
+    @property
+    def is_root(self):
+        return self.parent is None
+
 
 class User(AbstractUser, PermissionsMixin, BaseEntity):
     username = models.CharField("英文名/账号", max_length=30, unique=True)