...more importantly, this `put()` and the query above need to go together inside a datastore transaction.
example: https://github.com/snarfed/bridgy/blob/master/publish.py#L247
docs: https://developers.google.com/appengine/docs/python/datastore/transactions

i'm ok with a new `SyndicatedPost` method, a new top-level fn here, or a nested fn. up to you.