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".

Usage is only ever reported within one project. API key callers must pass project_id: the gateway strips the X-Project-Id header on this route, so the header does not work here.