Adds alerting stanza to application.yml with all AlertingProperties
fields backed by env-var overrides. Creates docs/alerting.md covering
six condition kinds (with example JSON), template variables, webhook
setup (Slack/PagerDuty examples), silence patterns, circuit-breaker
and retention troubleshooting, and Prometheus metrics reference.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
