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/api/payments.py

@@ -84,14 +84,25 @@ def intent(body: PaymentIntent, session_id=Depends(owner)):
                           AND created_at < now() - make_interval(mins => %s))))''',
                      (body.quote_id, CHALLENGE_MINUTES)).fetchone():
             raise HTTPException(409, 'A payment for this quote is already approved or in review')
+        method = body.method.model_dump()
         if body.method.type == 'pix':
-            existing = c.execute('''SELECT * FROM dtf_local.payment_intents WHERE quote_id=%s AND method='pix'
+            # One open PIX per quote: the same code until it expires, and a new
+            # one only after that, when the old code can no longer be paid.
+            # Serialised per quote, so two clicks never open two codes.
+            c.execute('SELECT pg_advisory_xact_lock(hashtext(%s))', ('pix:'+str(body.quote_id),))
+            existing = c.execute('''SELECT *, COALESCE((response->>'expires_at')::timestamptz <= now(), false) AS expired
+                FROM dtf_local.payment_intents WHERE quote_id=%s AND method='pix'
                 AND status='pending' ORDER BY created_at DESC LIMIT 1''', (body.quote_id,)).fetchone()
-            if existing:
+            if existing and not existing['expired']:
                 return existing['response']
+            if existing:
+                c.execute("UPDATE dtf_local.payment_intents SET status='expired', updated_at=now() WHERE id=%s",
+                          (existing['id'],))
+            method['attempt'] = c.execute('''SELECT count(*) AS n FROM dtf_local.payment_intents
+                WHERE quote_id=%s AND method='pix' ''', (body.quote_id,)).fetchone()['n'] + 1
         try:
             created = payment.create(str(body.quote_id), quote['approved']['total_cents'],
-                                     quote['approved']['customer'], body.method.model_dump())
+                                     quote['approved']['customer'], method)
         except ValueError as exc:
             raise HTTPException(422, str(exc))
         except Exception: