refactor: unify 4 isolated SQLAlchemy Base instances to database.models.Base

- database/import_models.py: 移除 ext.declarative.declarative_base，改用 from database.models import Base
- database/notification_models.py: 同上
- database/ppt_reports.py: 移除 orm.declarative_base，改用共用 Base
- database/vendor_models.py: 同上
- database/manager.py: 加入 4 個模型的 noqa import，確保 Base.metadata 完整管理所有資料表

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

database/vendor_models.py

@@ -6,11 +6,10 @@
 """
 
 from sqlalchemy import Column, Integer, String, DateTime, Boolean, ForeignKey, Numeric, Date, Text
-from sqlalchemy.orm import relationship, declarative_base
+from sqlalchemy.orm import relationship
+from database.models import Base
 from datetime import datetime
 
-Base = declarative_base()
-
 class VendorStockout(Base):
     """廠商缺貨表 - 儲存匯入的缺貨資料"""
     __tablename__ = 'vendor_stockout'