-- Theme B (P1-14): add Heartbeat.errorClass

ALTER TABLE `Heartbeat`
  ADD COLUMN `errorClass` VARCHAR(32) NULL;

-- Backfill: mark legacy down-rows so notification rules with an
-- errorClass condition don't silently miss them.
UPDATE `Heartbeat`
   SET `errorClass` = 'UNKNOWN'
 WHERE `status` = 'down'
   AND `errorClass` IS NULL;