On-call escalation policy: document and link to Mattermost + Zabbix #3
Description
Write the on-call escalation policy:\n1. Define severity levels (P1/P2/P3) with response time SLOs\n2. Map Zabbix trigger severities to escalation levels\n3. Document primary/secondary on-call rotation schedule template\n4. Configure Zabbix escalation action (notify → re-notify after N min → escalate)\n5. Add escalation policy to docs/05-guides/17-admin-runbook.md as a dedicated section