Optional webhook for critical dashboard regression alerts #49

Summary

AlertService only feeds the dashboard. No outbound Slack/webhook. Handoff docs still list SLA alerts as planned; a webhook is the smallest ops-shaped step.

Context

• docs/architecture/alerts.md
• Dashboard alerts UI (dashboard/static/app.js)
• scripts/uptime-ready-check.sh Slack notify pattern

Acceptance criteria

• Optional ALERT_WEBHOOK_URL (Slack incoming-webhook JSON is enough)
• Fires on new critical alerts with org id, code, severity
• Off by default; no production-validation requirement unless URL set
• Unit test with mocked HTTP client

Do not build a full notification platform.