Add the public /pay/robokassa/result callback proxy (rate-limited; forwards the
provider's form parameters to the backend intake, the single writer, and echoes
its "OK<InvId>" back to Robokassa) and the /pay/robokassa/{success,fail}
browser-return redirects into the app. Route /pay/* to the gateway in the
contour Caddyfile so the callback reaches the edge, not the landing catch-all.
The backend intake now returns the echo body as JSON for the gateway to relay.
Ilia Denisov
4f6c22d669