Store server IPs in one form (no brackets, short lower-case IPv6) and
write each address in the form its context needs:
- URLs and host:port (Sentinel, database URLs, SFTP backup download,
DNS servers, SMTP, webhook base URLs, links, health checks) get
brackets through formatHostForUrl().
- Traefik Host() rules use the address without brackets, so IPv6
domains no longer return 404; saved labels are fixed at deploy.
Caddy keeps the brackets.
- SSH keeps the plain address.
- Hetzner gives an IPv6 /64 network; IPv6-only servers now get the
server address (prefix::1) instead of the unusable prefix::64.
- sslip labels, registry image names with an IPv6 host, the API domain
list, allowlist matching and duplicate checks handle IPv6.
A migration normalizes stored IPv6 server addresses, repairs Hetzner
servers saved as ::64 that are not reachable, and adds brackets to
saved Sentinel URLs.
Add queued DNS checks with polling, status indicators, and notifications for application and service domains. Improve volume backup target labels and names.