Create a cart

Creates a new cart and adds inventory items to it in a single request. Holds the inventory for the customer. - `AssignedSeat` variants always require `quantity: 1` (each seat is unique) - `GeneralAdmission` and `Extra` variants support `quantity > 1` - All items must be from the same event + occurrence. - A cart cannot currently mix general admission and assigned seats. Use a separate cart for each. This is a limitation of how inventory is held today, not of the request format, and is expected to lift. Send a `currency` in the body to create the cart in that currency: the cart keeps pricing in it on later reads and the order settles in it. Omit it for base-currency prices. A currency this event is not sold in returns `400`; one it is sold in but that cannot be priced right now (no exchange rate) returns `503`. Send a `voucherCode` in the body to hold inventory that is hidden behind that code and to apply the code to the new cart in the same request. A well-formed code that does not match a voucher, or that cannot be applied to these items, is ignored rather than reported — the cart is created as if the field were omitted. A blank or over-length value is malformed and returns `400` like any other field. Send `previousCartId` when the customer re-selects seats: once the new cart is created, the previous cart is released so the customer holds one set of inventory instead of stacking a fresh hold on every selection. The release is best-effort and never fails the create — a cart that cannot be released is left alone.