refactor(ssh): decentralize SSH configuration to per-host services

Restructures SSH trust relationships from global to host-specific configuration
for better locality of concern and principle of least privilege.

Changes:
- Collapse nixos/common/global/ssh/ back to ssh.nix (single-file module)
- Move internal host trust (fuchsia/viridian) to per-host services/ssh/
- Split BorgBase known hosts by repository (li9kg944 for fuchsia, r7ag7x1w for viridian)
- Add viridian SSH server config to accept backup connections from fuchsia
- Add fuchsia borgbackup passphrase for offsite backups
- Configure viridian to create /srv/borg-repo/fuchsia for remote backups

This enables the 3-2-1 backup strategy with fuchsia backing up to both viridian
(onsite) and BorgBase (offsite) with proper SSH authentication.

nixos/fuchsia/services/default.nix

@@ -7,6 +7,7 @@
     ./pipewire
     ./printing
     ./snapper
+    ./ssh
     ./udev
     ./xserver
   ];