Returns aggregate statistics for administrative operations across all tenants (or filtered by tenant list). The counters cover every retained admin audit event in scope: the aggregation applies no time predicate, so summary.timeRange is all and the population is bounded only by the audit retention window. This endpoint has no fixed reporting period — for a bounded window call GET /api/v3/audit/platform/stats with startTime and endTime, which echoes the applied bounds back in the same field.