Records all four tender types. Only cash sets cash_given/change_given;
card, EFT and account settle outside the till, so cashup can reconcile
the physical drawer. Stock decrements for every tender including
account, since the goods leave on a delivery note.
client_ref makes checkout idempotent: an offline-queued sale that was
already committed server-side replays to the original record instead of
decrementing stock twice.
Co-Authored-By: Claude <noreply@anthropic.com>
