feat: PIX codes expire after 30 minutes, with a countdown

A PIX is created with a 30-minute date_of_expiration, and the PIX page
counts down to it. When it runs out the page says the code expired and
offers a new one. The API keeps one open code per quote: the same code
until it expires, then exactly one new attempt (serialised per quote, with
its own idempotency key), the old one marked expired.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/adapters.py

@@ -106,6 +106,11 @@ class FakePayment:
 
     def create(self, quote_id: str, total_cents: int, customer: dict, method: dict | None = None) -> dict:
         kind = (method or {}).get('type', 'pix')
+        if kind == 'pix':
+            from datetime import datetime, timedelta, timezone
+            expires = (datetime.now(timezone.utc) + timedelta(minutes=30)).isoformat(timespec='milliseconds')
+            return {'provider': 'fake', 'id': f"local-{quote_id}-pix-{(method or {}).get('attempt', 1)}",
+                    'status': 'pending', 'total_cents': total_cents, 'expires_at': expires}
         return {'provider': 'fake', 'id': f'local-{quote_id}-{kind}',
                 'status': 'pending', 'total_cents': total_cents}