The production compose hard-coded the fake payment adapter; it now takes
PAYMENT_ADAPTER and the MP_* settings from the stack's environment, so the
sandbox can run with test credentials. A signed notification about a payment
Mercado Pago does not have, such as the panel's "Simular notificação", is
acknowledged instead of answering 500 and being retried; any other lookup
failure still raises.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>