Reset the audience to the Pending state so the renewal worker
re-runs its unload. Use this to manually trigger a fresh build
when you don't want to wait for the next scheduled renew_time
tick, or to recover an audience that hit a transient Failed state.

Refresh does not re-validate or re-compile the filter, so it
succeeds even when the upstream schema service is unavailable.
It also does not change the filter, name, description, or any
other configuration. The response is the audience as-of the
reset, with status = Pending and task_id cleared.

Returns 409 Conflict when the audience is already mid-unload
(Syncing).