move deploy key to host config, add ralph and benjamin users

- Move forgejo-deploy pubkey from users/steffen to hosts/cryodev-main/
  (deploy key belongs to the host, not a user)
- Remove deploy key from steffen's authorized keys
- Add users ralph and benjamin (pubkeys pending)
- Register both new users in cryodev-main host config

hosts/cryodev-main/services/openssh.nix

@@ -12,6 +12,6 @@
 
   # Root SSH key for deploy-rs (key-only, no password)
   users.users.root.openssh.authorizedKeys.keyFiles = [
-    ../../../users/steffen/pubkeys/forgejo-deploy.pub
+    ../deploy-key.pub
   ];
 }