refactor(models): 将User.Groups的默认值逻辑迁移到BeforeCreate钩子

将Groups字段的默认值设置从gorm标签迁移到BeforeCreate钩子函数中，使默认值逻辑更灵活可控

Author: woodchen-ink

backend/models/user.go

@@ -10,13 +10,21 @@ type User struct {
 	ID        uint           `json:"id" gorm:"primaryKey"`
 	Username  string         `json:"username" gorm:"not null;unique"`
 	Email     string         `json:"email" gorm:"not null;type:varchar(191)"`
-	Role      string         `json:"role" gorm:"not null;default:user"`    // admin or user (legacy)
-	Groups    string         `json:"groups" gorm:"type:text;default:'t0'"` // CZL Connect权限组: t0,t1,t2,t3,t4,t5,viewer,admin
+	Role      string         `json:"role" gorm:"not null;default:user"` // admin or user (legacy)
+	Groups    string         `json:"groups" gorm:"type:text"`           // CZL Connect权限组: t0,t1,t2,t3,t4,t5,viewer,admin
 	CreatedAt time.Time      `json:"created_at" gorm:"autoCreateTime"`
 	UpdatedAt time.Time      `json:"updated_at" gorm:"autoUpdateTime"`
 	DeletedAt gorm.DeletedAt `json:"-" gorm:"index"`
 }
 
+// BeforeCreate GORM hook: 在创建用户前设置默认值
+func (u *User) BeforeCreate(tx *gorm.DB) error {
+	if u.Groups == "" {
+		u.Groups = "t0"
+	}
+	return nil
+}
+
 type Session struct {
 	ID        string         `json:"id" gorm:"primaryKey"`
 	UserID    uint           `json:"user_id" gorm:"not null"`