Dead-man's-switch heartbeat for ntfy. Timer every 300s, publishes test msg to https://ntfy-prod.internal (forge), pings healthchecks.io on success/fail. Detects ntfy/forge/ironclad outage independent of Uptime Kuma (which alerts via ntfy itself). Runs /usr/services/ntfy-heartbeat-prod/start.sh as gandalf. Hardening: base4 (NoNewPrivileges, PrivateTmp, ProtectSystem=true, ProtectHome=false).