Book a held quote as a firm rate the bank honours until it expires (idempotent under its Idempotency-Key)

Initiate permission on the source account, and an Idempotency-Key. Booking makes the quote a firm rate the bank honours until it expires, so a service carrying out a customer instruction can rely on the price before it moves any money. Booking is idempotent under its key: the same key answers the booking already made with 200, a different key on an already booked quote is 409, a cancelled or expired quote is 409, and a quote whose time has passed is accounts.fx_quote_expired (409).