OptionalmessageOptionalpayOptional caller-supplied "tap to pay" link for the recipient SMS. It is
shortened before the row is inserted and the short id is stored on the
row as payShortUrlId; the SMS handler composes the link from it.
Customer-scoped identity id of the requester.
Requester's phone in any common format, stored in E.164 — shown to the
recipient; the self-send guard compares it against recipientPhone.
Human-decimal amount (e.g. 1.31), per the financial-services BigNumber convention. Persisted as the decimal value.