Signal alert webhook receiver.

• Health: GET /health
• Alerts: POST /webhook/alert (header x-alert-token, JSON body)
• Coolify (no custom headers): POST /webhook/coolify?token=… (same secret as WEBHOOK_TOKEN)
• Grafana: Extra header x-alert-token + POST /webhook/grafana (default Grafana JSON is fine)
• Same token for /webhook/kuma, …