feat(infra): add alerting, network segmentation, and ops docs (Steps 19-24)

- Prometheus alert rules (host, container, API, Celery, target-down)
- Alertmanager with email routing (critical 1h, warning 4h repeat)
- Docker network segmentation (frontend/backend/monitoring)
- Incident response runbook with 8 copy-paste runbooks
- Environment variables reference (55+ vars documented)
- Hetzner setup docs updated with Steps 19-24
- Launch readiness updated with Feb 2026 infrastructure status

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

mkdocs.yml

@@ -215,6 +215,7 @@ nav:
       - Gitea CI/CD: deployment/gitea.md
       - Hetzner Server Setup: deployment/hetzner-server-setup.md
       - Environment Variables: deployment/environment.md
+      - Incident Response: deployment/incident-response.md
       - Stripe Integration: deployment/stripe-integration.md
 
     - Operations: