fix: resolve fatal startup error in SysUser model caused by invalid column type

inventory-backend/app/models/system.py

@@ -22,7 +22,7 @@ class SysUser(db.Model):
     role = db.Column(db.String(50))
     status = db.Column(db.String(20), default='active')
     password_hash = db.Column(db.Text)
-    preferences = db.Column(db.JSON, default=dict)  # 用户偏好/个性化配置（如齐套监控列表）
+    preferences = db.Column(db.JSON, nullable=True)  # 用户偏好（如齐套监控列表）；Python层用 or {} 兜底
     created_at = db.Column(db.DateTime, default=beijing_time)
 
     def set_password(self, password):