feat: add partial return support with returned_quantity tracking

inventory-backend/app/models/transaction.py

@@ -13,6 +13,7 @@ class TransBorrow(db.Model):
     stock_id = db.Column(db.Integer)
     barcode = db.Column(db.String(100))
     quantity = db.Column(db.Numeric(19, 4))
+    returned_quantity = db.Column(db.Numeric(19, 4), default=0)
     borrower_name = db.Column(db.String(100))
     borrow_time = db.Column(db.DateTime, default=beijing_time)
     borrow_signature = db.Column(db.Text)
@@ -26,6 +27,9 @@ class TransBorrow(db.Model):
     remark = db.Column(db.Text)
 
     def to_dict(self):
+        returned_qty = float(self.returned_quantity) if self.returned_quantity is not None else 0
+        total_qty = float(self.quantity) if self.quantity is not None else 0
+        pending_qty = total_qty - returned_qty
         return {
             'id': self.id,
             'borrow_no': self.borrow_no,
@@ -33,7 +37,9 @@ class TransBorrow(db.Model):
             'source_table': self.source_table,
             'stock_id': self.stock_id,
             'barcode': self.barcode,
-            'quantity': float(self.quantity) if self.quantity is not None else None,
+            'quantity': total_qty,
+            'returned_quantity': returned_qty,
+            'pending_quantity': pending_qty,
             'borrower_name': self.borrower_name,
             'borrow_time': self.borrow_time.strftime('%Y-%m-%d %H:%M') if self.borrow_time else None,
             'borrow_signature': self.borrow_signature,