Get the member inventory revision

Returns an opaque revision that changes whenever what organization members can use may have changed: after any write to providers, plugins, marketplaces, connections, teams, members or their access, after a feature flag change, and on every deploy. Clients compare it with the revision they last loaded and skip reloading their inventory when it is unchanged. A null revision means it cannot be determined, so the client reloads. It describes the organization, not the caller, and reveals nothing else.