fix: add extend_existing=True to StockAdjustment model to resolve MetaData conflict

inventory-backend/app/models/stock/adjustment.py

@@ -9,6 +9,7 @@ class StockAdjustment(db.Model):
     用于记录财务/主管手动发起的库存修正
     """
     __tablename__ = 'stock_adjustment'
+    __table_args__ = {'extend_existing': True}
 
     id = db.Column(db.Integer, primary_key=True)
     # 单号，如 ADJ-YYYYMMDD-XXXX