refactor(ssh): centralize identity file logic and optimize host selection

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.

modules/system/users.nix

@@ -156,6 +156,7 @@
               createHome = false;
               homeManager = { enable = false; module = null; };
               ssh.inbound.enable = true;
+              ssh.inbound.authorizedHosts = "all";
             };
           };