fix(deploy): wire the Robokassa direct rail into the prod deploy + rollback

The Robokassa credentials reached only the test contour (ci.yaml → TEST_
secrets). The prod-deploy / prod-rollback workflows and write-prod-env.sh
never rendered BACKEND_ROBOKASSA_*, so on prod the shop login was empty and
the direct RUB rail stayed disabled — the PROD_BACKEND_ROBOKASSA_* secrets
went nowhere.

Export the shop login + Password1/Password2 (secrets) and a
BACKEND_ROBOKASSA_TEST flag (a variable, so go-live is a flag flip not a
secret rotation) from both prod workflows, and emit the four ROBOKASSA_*
vars from write-prod-env.sh (the shared deploy/rollback env renderer) so a
rollback keeps the rail up. The compose already maps ROBOKASSA_* →
BACKEND_ROBOKASSA_*. Document the new secrets/variable in the deploy README
and .env.example. Password3 (Robokassa's JWT-invoice API) is unused.

.gitea/workflows/prod-rollback.yaml

@@ -61,6 +61,12 @@ jobs:
       # the SAME env.sh (email / VK login / Grafana alerts survive a rollback). TELEGRAM_MINIAPP_URL
       # and GRAFANA_ROOT_URL are derived from PUBLIC_BASE_URL in deploy/write-prod-env.sh.
       GATEWAY_VK_APP_SECRET: ${{ secrets.GATEWAY_VK_APP_SECRET }}
+      # Robokassa direct-rail: the rollback re-renders the same runtime env (write-prod-env.sh), so
+      # it must carry the same credentials or the direct rail goes dark after a rollback.
+      ROBOKASSA_MERCHANT_LOGIN: ${{ secrets.PROD_BACKEND_ROBOKASSA_MERCHANT_LOGIN }}
+      ROBOKASSA_PASSWORD1: ${{ secrets.PROD_BACKEND_ROBOKASSA_PASSWORD1 }}
+      ROBOKASSA_PASSWORD2: ${{ secrets.PROD_BACKEND_ROBOKASSA_PASSWORD2 }}
+      ROBOKASSA_TEST: ${{ vars.PROD_BACKEND_ROBOKASSA_TEST }}
       VITE_VK_APP_ID: ${{ vars.VITE_VK_APP_ID }}
       GATEWAY_VK_ID_CLIENT_SECRET: ${{ secrets.GATEWAY_VK_ID_CLIENT_SECRET }}
       GATEWAY_HONEYTOKEN: ${{ secrets.PROD_GATEWAY_HONEYTOKEN }}