feat: let production select Mercado Pago and acknowledge simulated notifications

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>

app/mercadopago.py

@@ -134,7 +134,15 @@ class MercadoPagoPayment:
         payment_id = str((query or {}).get('data.id') or (data.get('data') or {}).get('id') or '')
         if not payment_id.isdigit():
             return None
-        payment = self.lookup(payment_id)
+        try:
+            payment = self.lookup(payment_id)
+        except httpx.HTTPStatusError as error:
+            # Signed by Mercado Pago but about no payment of ours, such as the
+            # panel's "Simular notificação". Anything else is raised so that a
+            # real notification is retried.
+            if error.response.status_code == 404:
+                return None
+            raise
         return event_from_payment(payment, notification_id=str(data.get('id', '')))