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/prod-deploy.yaml

@@ -99,7 +99,7 @@ jobs:
       ADMIN_EMAIL: ${{ vars.PROD_ADMIN_EMAIL }}
       SMTP_RELAY_SERVICE_FROM: ${{ vars.PROD_SMTP_RELAY_SERVICE_FROM }}
       SERVICE_EMAIL: ${{ vars.PROD_SERVICE_EMAIL }}
-      GRAFANA_SMTP_HOST: ${{ vars.PROD_GRAFANA_SMTP_HOST }}
+      GRAFANA_SMTP_PORT: ${{ vars.PROD_GRAFANA_SMTP_PORT }}
       GF_SMTP_ENABLED: ${{ vars.PROD_GF_SMTP_ENABLED }}
       PUBLIC_BASE_URL: ${{ vars.PROD_PUBLIC_BASE_URL }}
       PROD_BOTLINK_CA: ${{ secrets.PROD_BOTLINK_CA }}
@@ -168,7 +168,7 @@ jobs:
           export ADMIN_EMAIL='$ADMIN_EMAIL'
           export SMTP_RELAY_SERVICE_FROM='$SMTP_RELAY_SERVICE_FROM'
           export SERVICE_EMAIL='$SERVICE_EMAIL'
-          export GRAFANA_SMTP_HOST='$GRAFANA_SMTP_HOST'
+          export GRAFANA_SMTP_PORT='$GRAFANA_SMTP_PORT'
           export GF_SMTP_ENABLED='$GF_SMTP_ENABLED'
           export PUBLIC_BASE_URL='$PUBLIC_BASE_URL'
           export GATEWAY_ABUSE_BAN_ENABLED='true'