Pause an audience so it stops auto-renewing on its scheduled renew_time. The current schedule is stashed in previous_renew_time and restored on resume; paused_at is stamped and pause_reason is set to manual.

Pausing does not cancel an in-flight unload, clear results, or change the filter. A paused audience can still be downloaded. It cannot be refreshed until it is resumed: the renewal worker skips paused rows, so a queued rebuild would never be claimed, and refresh returns 409 with pause_reason rather than accepting work that cannot run.

Returns 409 Conflict when the audience is already paused.