nixos/syncthing: make it easier to debug incorrect IDs

In case of an incorrect ID, the behavior of this script
is quite tricky. See:

https://github.com/NixOS/nixpkgs/issues/326704#issuecomment-2730819822

for more info.

nixos/modules/services/networking/syncthing.nix

@@ -121,6 +121,7 @@ let
           '[.[].${s.GET_IdAttrName}] - $new_ids | .[]'
         )"
         for id in ''${stale_${conf_type}_ids}; do
+          >&2 echo "Deleting stale device: $id"
           curl -X DELETE ${s.baseAddress}/$id
         done
       ''