Issue #3609963: Do not invent a checkout amount; a zero-amount payment starts no checkout

A free booking that needs a card on file was set up with an invented amount: WorldlineGateway::initiate() substituted a configured verification_amount (default €1) for a zero-amount checkout, and the finalizer then recorded the row as captured for €0. The row matched neither Worldline's amount nor its state.

kessai now never invents a checkout amount. It authorizes exactly the payment's amount and records the state Worldline reports, so the row mirrors Worldline. A zero-amount payment starts no checkout; a caller that needs a card on file for a free booking authorizes a real amount (e.g. the deposit) through the payment amount, and the workflow settles that authorization (capturing what is owed, or cancelling it).

Removes the verification_amount setting (form, schema, default) and the finalizer's zero-amount branch. Kernel tests updated; the amount-mismatch branch keeps the structured release-error logging coverage.