fix order-transaction one-to-one relation

[valentino-sm]

fix sqlalchemy:latest warning:

SAWarning: relationship 'Order.transaction' will copy column orders.order_id to column transactions.order_id, which conflicts with relationship(s): 'Transaction.order' (copies orders.order_id to transactions.order_id). If this is not the intention, consider if these relationships should be linked with back_populates, or if viewonly=True should be applied to one or more if they are read-only.

[Steffo99]

That's a good fix, but for more clarity I'd prefer if back_populates was used. :)