style(api): drop non-null assertion in refund amount

Replace `input.amount!` with `input.amount ?? Number(payment.amount)`,
which is behaviour-identical (undefined amount = full refund = full
amount) but satisfies lint/style/noNonNullAssertion.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/api/src/payments/stripe/stripe.service.ts

@@ -394,7 +394,7 @@ export class StripeService {
       paymentId: payment.id,
       userId: payment.userId,
       stripeRefundId: refund.id,
-      amount: isFullRefund ? Number(payment.amount) : input.amount!,
+      amount: input.amount ?? Number(payment.amount),
       isFullRefund,
       newStatus,
       currency: payment.currency,