Reserve Basket items

Soft-holds availability for every item in the basket so the customer can complete payment without losing their slot. Reserved items expire after a short window (the response's items_reserved_until indicates the deadline) and are automatically released if the basket isn't submitted in time.

A customer must be attached to the basket (POST /shop/basket/{basketId}/customer) before items can be reserved — call this immediately before sending the customer to the payment step.

Returns 400 Bad Request if one or more items have become unavailable since they were added; the response message identifies the conflict.