test: add webhook alert integration tests

[br413]

Summary

WebhookAlertChannel is implemented and wired via CLI --webhook-url, but tests only cover console and file channels.

Acceptance criteria

• Unit tests mock \urllib.request.urlopen\ for successful POST payload
• Test delivery failure raises \RuntimeError\ with \webhook delivery failed`n- [ ] AlertRouter fans out failed checks to webhook channel
• README capability checkbox checked

Part of 90-day portfolio plan (Week 3).