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.

deploy/docker-compose.yml

@@ -501,11 +501,12 @@ services:
       # caddy's Basic-Auth and re-prompts for the password on every dashboard; the
       # dashboards poll and do not need Live.
       GF_LIVE_MAX_CONNECTIONS: "0"
-      # SMTP for alert emails, reusing the shared relay credentials on its STARTTLS host:port
-      # (GRAFANA_SMTP_HOST — Grafana's client speaks STARTTLS, not the backend's implicit-TLS
-      # port), the SERVICE From, and the SERVICE recipient. Disabled unless GF_SMTP_ENABLED.
+      # SMTP for alert emails, reusing the shared relay host + credentials + the SERVICE
+      # From/recipient. Grafana's client speaks STARTTLS (not the backend's implicit-TLS
+      # port), so it dials the relay host on its STARTTLS port (GRAFANA_SMTP_PORT).
+      # Disabled unless GF_SMTP_ENABLED.
       GF_SMTP_ENABLED: ${GF_SMTP_ENABLED:-false}
-      GF_SMTP_HOST: ${GRAFANA_SMTP_HOST:-}
+      GF_SMTP_HOST: ${SMTP_RELAY_HOST:-}:${GRAFANA_SMTP_PORT:-}
       GF_SMTP_USER: ${SMTP_RELAY_USER:-}
       GF_SMTP_PASSWORD: ${SMTP_RELAY_PASS:-}
       GF_SMTP_FROM_ADDRESS: ${SMTP_RELAY_SERVICE_FROM:-}