Refactor the SSH configuration generation to reduce duplication and
improve efficiency.
- Extract `IdentityFile` line generation into a shared variable in
`ssh-inventory.nix` to ensure consistency between per-host and
gitea blocks.
- Optimize `selectedHosts` resolution in `home-manager/default.nix`
by reusing `activeHosts` when the "all" wildcard is used.
- Tighten IPv4 regex matching in `ssh-inventory.nix` using anchors.
- Update `users-merge.nix` to propagate `extraIdentities` to the
rbw outbound configuration.
- Set default `authorizedHosts` to "all" for inbound SSH users.
Olivier