- Add regex filter input to the Alerts dashboard that filters displayed
hosts on every keystroke; invalid regex turns the border red
- Initialise the filter from ?filter= in the URL query string
- Change _build_url() to produce /alerts?filter=<hostname> so
notification links (Pushover, email, Matrix, etc.) land on the
alerts page pre-filtered to the alerting host
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>