Add Product.last_sold to schema

hopefully that's a good idea..

corepos/db/office_op/model.py

@@ -423,6 +423,9 @@ class Product(Base):
 
     current_origin_id = sa.Column(sa.Integer(), nullable=True, default=0)
 
+    # TODO: some older DB's might not have this?  guess we'll see
+    last_sold = sa.Column(sa.DateTime(), nullable=True)
+
     like_code = association_proxy(
         '_like_code', 'like_code',
         creator=lambda lc: ProductLikeCode(like_code=lc),