0.12.0 - Optional webhooks

Added

• Optional webhooks (live on the P2Flux API since 15 October 2026). Add notify_url (a public
  https URL) to createPayment() or createPaymentLink() and P2Flux POSTs payment.paid,
  subscription.charged or subscription.updated there, retrying for about 18 hours. Without it
  nothing changes. The response repeats notify_url only when the payment or link is watched.
  (https://p2flux.com/docs/webhooks.html)
• handleNotification($body, $expect) - confirms a notification with the API before you fulfil:
  ['reference' => ...] for a payment, ['link_id' => ..., 'manage' => ...] for a link. Returns
  confirmed, a dedupe_key built from the verified payment (never the event id), and throws
  P2FluxException when the answer is not final yet (answer 5xx; it is sent again).