The customer was charged twice after a 30-second timeout

• Support tickets: customers see two identical card charges for one order, a few seconds apart.
• The mobile app showed a spinner for 30 s, then "Something went wrong — please try again". Tapping again "worked".
• It affects ~0.3% of checkouts, and only during the evening peak (19:00–21:00).
• The payments table has two rows for the same order_id, each with a different provider charge id.

# api-gateway   (upstream timeout: 30 000 ms)
19:42:07.114  req=7f3a  POST /orders/8841/pay  → payment-svc
19:42:37.120  req=7f3a  504 upstream timeout after 30000 ms
19:42:38.905  req=9c21  POST /orders/8841/pay  → payment-svc     (same device, automatic retry)
19:42:41.330  req=9c21  200 OK  (3.4 s)

# payment-svc
19:42:07.118  req=7f3a  order=8841  provider.charge(amount=4990) start
19:42:38.910  req=9c21  order=8841  provider.charge(amount=4990) start
19:42:39.671  req=7f3a  order=8841  provider.charge ok  psp_id=ch_1Q8x   (32.5 s)
19:42:39.680  req=7f3a  order=8841  INSERT INTO payments ... status=captured
19:42:41.325  req=9c21  order=8841  provider.charge ok  psp_id=ch_1Q8y   (2.4 s)
19:42:41.328  req=9c21  order=8841  INSERT INTO payments ... status=captured

# payments
id    order_id  psp_id    amount  created_at
501   8841      ch_1Q8x   4990    19:42:39
502   8841      ch_1Q8y   4990    19:42:41