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/import_models.py

@@ -5,7 +5,7 @@
 """
 
 from sqlalchemy import Column, Integer, String, DateTime, Text, Float
-from sqlalchemy.ext.declarative import declarative_base
+from database.models import Base
 from datetime import datetime
 import pytz
 
@@ -18,9 +18,6 @@ def taipei_now():
     return datetime.now(TAIPEI_TZ).replace(tzinfo=None)
 
 
-Base = declarative_base()
-
-
 class ImportJob(Base):
     """匯入任務模型"""
     __tablename__ = 'import_jobs'