Returns export and campaign usage for a batch of audiences, so a list view can show which ones are in use without a request per row.

Every requested id comes back, in the order requested. An audience with no live export is returned with zero counts and in_use false, which also covers ids that no longer exist, so a missing entry never means "not found".

The project is taken from the authenticated context rather than a parameter, so usage is only ever reported within the caller's own project.