feat(payments): direct-rail order + intake handlers, config and reaper

Wire the Robokassa direct rail into the backend transport. POST
/api/v1/user/wallet/order (walletGate + a D36 confirmed-email gate for the
direct rail) opens a pending order and returns the signed Robokassa payment
URL. The internal, gateway-only /payments/robokassa/result endpoint verifies
the Result signature, credits the matched order exactly once via Fund (honoured
even if expired), records a succeeded payment event, and answers Robokassa's
"OK<InvId>". Add the Robokassa env config, an account HasConfirmedEmail check
(D36), the payment_events writer, and a periodic pending-order reaper. The
routes register only when a Robokassa merchant login is configured.

backend/internal/payments/service_intake.go

@@ -71,3 +71,9 @@ func (s *Service) ExpireOrders(ctx context.Context) (int, error) {
 	}
 	return s.store.expirePending(ctx, ttl, s.clock())
 }
+
+// RecordPaymentEvent appends a payment lifecycle event (succeeded/failed/refunded) for the
+// dispatcher to deliver to the user (live stream, botlink or email).
+func (s *Service) RecordPaymentEvent(ctx context.Context, accountID uuid.UUID, orderID *uuid.UUID, eventType string, payload []byte) error {
+	return s.store.insertPaymentEvent(ctx, accountID, orderID, eventType, payload, s.clock())
+}