fix(deploy): Grafana SMTP reuses relay host + explicit GRAFANA_SMTP_PORT

Drop the separate GRAFANA_SMTP_HOST (it duplicated SMTP_RELAY_HOST): Grafana differs from
the backend only in needing the STARTTLS port, so GF_SMTP_HOST is composed from
SMTP_RELAY_HOST + GRAFANA_SMTP_PORT (an explicit per-contour var, no magic default), reusing
SMTP_RELAY_USER/PASS. Wired through ci/prod/.env.example/README.

.gitea/workflows/ci.yaml

@@ -347,7 +347,7 @@ jobs:
           ADMIN_EMAIL: ${{ vars.TEST_ADMIN_EMAIL }}
           SMTP_RELAY_SERVICE_FROM: ${{ vars.TEST_SMTP_RELAY_SERVICE_FROM }}
           SERVICE_EMAIL: ${{ vars.TEST_SERVICE_EMAIL }}
-          GRAFANA_SMTP_HOST: ${{ vars.TEST_GRAFANA_SMTP_HOST }}
+          GRAFANA_SMTP_PORT: ${{ vars.TEST_GRAFANA_SMTP_PORT }}
           GF_SMTP_ENABLED: ${{ vars.TEST_GF_SMTP_ENABLED }}
           # Canonical public origin for links in the email (this contour's URL);
           # required by the backend whenever SMTP_RELAY_HOST is set.