Optional webhook for critical dashboard regression alerts

## 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.

Read docs/architecture/alerts.md and inspect the dashboard alert flow in dashboard/static/app.js, then compare the Slack notification pattern in scripts/uptime-ready-check.sh. Locate AlertService and the critical-alert creation path; completion means an unset ALERT_WEBHOOK_URL changes nothing, while a set URL sends the required JSON, with mocked-HTTP unit coverage.

Written by the indexing model from the issue text.