createPendingOrder validates availability, snapshots titles, artists
and prices, and issues LR-YYYY-NNNN references; beginCheckout builds
the Stripe session from the snapshots; completeFromCheckoutSession is
idempotent across webhook retries and applies the no-holds policy —
when two buyers pay for the same work the first completed payment wins
and the second is refunded in full. Expiry, pickup, refund and cancel
transitions round out the lifecycle for the CMS.

Signed-off-by: Simon Mundy <simon.mundy@peptolab.com>