Notifications dropped after rollback

v210 of the `notifications` consumer is rolled back at 16:00 after a latency regression. The rollback is rolling, and the deploy tool reports 'complete, 100% on v209' at 16:05. But a subset of notifications keep getting dropped silently for another 30 minutes. Context: v210 had introduced a new compaction format for the per-user dedup state it stores in a shared Redis hash, and it had been writing that new format for the ~25 minutes it was live. v209's reader expects the old format and treats unparseable dedup entries as 'already sent,' so it skips them. Dashboards: consumer lag is zero, no errors thrown (the skip is silent), Redis is healthy. Triage and explain why a 'complete' rollback didn't stop the bleeding, then mitigate.