13ea341ef8
- migrate_group() REPLACEs the whole flat row; columns without a native row fell back to their DDL default (session_count / account_flags DEFAULT 0), so every backfilled user got invented zeros. Keys with no native row are now written as NULL; required NOT NULL columns keep their default. - Covers the cron batch path (migrate_group_batch) and clears stale flat values on re-run. - Verify: EntityMigrationEngineDefaultsTest (3 RED on main); dev36 user/activity backfill: checker extra 123 → 0 for session_count and account_flags, miss/diff unchanged. - PR: PR-066