Adds dashed:check-integrations-health command that runs hourly via the
scheduler. For every registered integration x configured site it probes
the health_check closure, persists the last-known status in a
Customsetting, and on real transitions (Connected -> not-Connected, or
recovery back to Connected) sends IntegrationStatusChangedMail to all
admins via AdminNotifier.

- One mail per transition, never duplicate spam while the status is
  unchanged
- Skips no-op transitions like Misconfigured <-> Failing (no extra
  signal) and Disabled (explicitly turned off by admin)
- Mail view shows old/new status, error message, slug, site, and a
  link to the integrations dashboard

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>