Summary

Adds webhook. Each monitor can now have a webhook URL
configured from its detail page; when a check transitions between up and
down, pingtower POSTs a JSON payload to that URL.

Behaviour

• Fires only on status transitions (up → down or down → up). No noise on every successful poll.
• Never fires on the very first poll — guarded by check.LastStatus != "" so adding a new monitor doesn't trigger a false alert.
• Fire-and-forget delivery with a 10-second context timeout. Failures are logged but never block the polling loop.

Payload

{
  "check_id": "...",
  "name": "...",
  "url": "https://example.com",
  "status": "down",
  "previous_status": "up",
  "status_code": 503,
  "response_ms": 412,
  "checked_at": "2026-05-04T10:24:11Z"
}