This is really clever - letting this be a O(webhooks) operation, without much memory overhead.  I realize that the sort order in cassandra can probably  be depended upon (we are depending upon it for pagination, for instance) - and an experiment bears out this assumption.  I haven't been able to find something in the cassandra documentation that documents this behavior exactly though - if you have it, would it be possible to link it here in the comment?

I'd feel a lot better if this were exact documented behavior (I've found references to the sort order here and there, but nothing outright, like "data will be sorted in order of the composite keys" or something), else it would feel somewhat fragile.