Fixes backup system authentication and hostname resolution issues.
Changes:
- Change PermitRootLogin from "no" to "prohibit-password" in global SSH config
(allows key-based root login for host-to-host backups while blocking passwords)
- Update fuchsia onsite backup to use viridian.home.arpa FQDN instead of shortname
- Update SSH knownHosts to use FQDNs (fuchsia.home.arpa, viridian.home.arpa)
(system-level config uses FQDNs, user shortcuts remain in home-manager)
This enables the complete 3-2-1 backup strategy with automated backups working
correctly between fuchsia and viridian, and fuchsia to BorgBase.