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.
