Starts a new fulfillment workflow for an existing completed order,
replacing the visit date with new_date. Supports idempotency via
the Idempotency-Key header.

The original order must be in COMPLETED status and must belong to
the authenticated user and their campaign. The old tickets are voided
with the provider on a best-effort basis — if voiding fails the new
order still proceeds and the old tickets will expire naturally.

The response is synchronous and returns only the new fulfillment order
ID and the requested date. The actual ticket issuance is asynchronous.
Poll GET /fulfillment/orders/{order_id} (using the checkout order_id
of the new order) to retrieve tickets once the workflow completes.