reference_id (the gateway's settled-transaction ref) was not unique, so the
same successful callback — or a RefNum replayed onto another order — could
credit twice. Add a unique index (NULL until success, so pending/failed rows
don't collide) and an application-level pre-check in the callback that fails the
payment if the reference already belongs to another order. The unique index is
the hard backstop behind the check.
Regression: PaymentCallbackAmountTest::testReplayedGatewayReferenceIsRejected.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
