- Backfill batches are scheduled with (entity, group) args; the health
panel looked for (entity) alone, never saw one and kept offering to start
a backfill, which resets the group's checkpoint
- is_backfill_active() checks every group of the entity
- Unit bootstrap: wp_next_scheduled() stub honours hook + args
- Verify: EntityHealthBackfillTest (2 new); unit 711, phpcs 0, stan OK
- PR: PR-137 (manual F-11)