Cancelling a confirmed order leaves the stock reserved

The catalogue is a Flask service in src/bookshop. Orders live in

src/bookshop/orders.py and the stock they act on lives in

src/bookshop/catalog.py; both modules document their rules, and the rules are

the specification. Read them first.

The invariant the service promises is:

• confirm reserves the stock every line needs;
• ship moves reserved stock out of the catalogue;
• cancel returns reserved stock to the catalogue;
• the available stock is always stock_on_hand - reserved.

One of those promises is broken. Find which one, and repair it.

What to check

Write down the stock counts before and after each transition and compare them

with what the docstrings promise. Do it for both kinds of cancellation: an order

that was never confirmed, and an order that was. Then do it through the HTTP

routes as well, because a route is allowed to be wrong in a way the service is

not.

Boundaries

• Do not change the transition table: which statuses may follow which stays as

documented.

• Do not modify the repository test suite under tests/.
• The public API of the bookshop package stays as it is.
• The repository's own test suite has to stay green after your change.