Issue #3623412: Follow kessai: name a payment account by its uuid, and call it an account

kessai renames two methods yoyaku_payment calls: getAvailableGateways(), where the booking payment form offers the accounts a site can pay into, and isGatewayAvailable(), in the resolver's availability check. Neither exists once the rename lands, so the form and every resolved payment fatal on a site that has both installed.

These three land together, in any order but within the same deploy:

• kessai !81 (#3621937)
• orchestra #3623411
• this one

kessai is pre-1.0 and deletes rather than deprecates, so there is no alias to soften the gap.

What changes

The two call sites, and the prose beside them.

What does not

yoyaku's own naming — PaymentGatewayResolver, getGatewayForOrder(), getGatewayForTenant() and the action setting keyed gateway — says gateway and means account too. It moves with the stored value rather than ahead of it: the key needs a migration, and renaming the interface first would leave it saying one thing and the config another.