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/lib/users-merge.nix

@@ -60,6 +60,7 @@
           chiasson.ssh.outbound.rbw.user = name;
           chiasson.ssh.outbound.rbw.hosts =
             if (outboundCfg.hosts or "all") == "all" then [ "all" ] else outboundCfg.hosts;
+          chiasson.ssh.outbound.rbw.extraIdentities = outboundCfg.extraIdentities or [ ];
         };
 
       mkHmUserModule =