Callback when added a ZIM entry for an image is not working

[benoit74]

In #367, I failed to make the call to once_done in src/sotoki/utils/imager.py be handled by the expected callback in add_item_for. I called it "manually" directly after add_item_for, which is not 100% accurate.

It looked like the callback was never called, despite the item being added to the ZIM and the same kind of call succeeding in other situations.

I failed so far to nail the issue down.

[benoit74]

Postponing, I really do not achieve to reproduce conditions which lead to callback not being called.