Delete an unsubscribe group

If the group has 0 active suppressions, hard-deletes the row. With force=true, soft-deletes all active suppressions first (status → removed, group_id cleared, removed audit event per block) in a single transaction, then hard-deletes the group. Without force and active suppressions present → 409. Audit trail is preserved. force only accepts the string "true" or boolean true; all other values are false.