fix: failed payments no longer count towards the daily limit

[AxelHamburch]

Problem

In lnurl_callback the hit is marked as spent with the full invoice amount before pay_invoice is attempted. When the payment then fails (e.g. the invoice amount exceeds tx_limit, so pay_invoice raises), the amount stays recorded in the hit.

On the next tap, api_scan sums all of today's hits — including failed ones — and rejects the card with LNURLW error: Max daily limit spent. even though no payment ever happened. A single over-limit attempt can lock the card for the rest of the day.

Changes

• Validate the invoice amount against tx_limit before marking the hit as spent. A rejected attempt no longer consumes the daily limit. The error message keeps the existing Payment failed - Invoice amount ... too high. Max allowed: ... format.
• Enforce the daily limit in the callback. The scan-time check runs before the amount is known, so a single payment could previously exceed the daily limit. The callback now checks spent_today + amount > daily_limit.
• Roll back the amount when pay_invoice raises. The hit amount is reset to 0 so the failed attempt doesn't count towards the daily limit. The hit stays spent, so the same k1 cannot be claimed again.

Testing

Reproduced and verified on an LNbits instance with a physical Bolt Card on a PoS terminal:

• Invoice above tx_limit → rejected with the expected error, card keeps working on the next tap (previously: blocked with "Max daily limit spent.")
• Invoice within limits → payment succeeds as before