feat(security): implement metadata redaction and sops-nix migration

Migrated authorized SSH keys and personal metadata (emails, tokens) to sops-nix to prevent infrastructure fingerprinting. Introduced centralized secrets module with placeholder fallbacks.

users/backup/default.nix

@@ -12,9 +12,8 @@ in {
       extraGroups = [
         "docker" # Allow access to the docker socket.
       ];
-      openssh.authorizedKeys.keys = [
-        # Hugo
-        "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAICms6vjhE9kOlqV5GBPGInwUHAfCSVHLI2Gtzee0VXPh"
+      openssh.authorizedKeys.keyFiles = [
+        config.sops.secrets.user_keys_backup.path
       ];
     };
   };