fix: 修复 certificate.py 的 Base 导入路径

backend/app/models/certificate.py

@@ -10,7 +10,7 @@ from typing import Optional
 from sqlalchemy import Column, Integer, String, Text, Boolean, DateTime, ForeignKey, Enum as SQLEnum, DECIMAL, JSON
 from sqlalchemy.orm import relationship
 
-from app.core.database import Base
+from app.models.base import Base
 
 
 class CertificateType(str, Enum):