Recount a segment

Triggers a record count computation for the segment. Runs a fresh computed count for every subtype: filter segments count directly against their own filter definition and update record_count; matched/similarity/propensity segments run the same composed count used by audience/delivery counting (a 1-membership composition), rather than echoing a possibly-stale stored count. This operation updates the saved count without requesting demographic breakdowns. The total aggregation privacy floor still applies. Records are not purchased or exported; backend compute telemetry is recorded. Requires 'discovery' scope.