The Great Queue Illusion

Book vs Prod micro-sim. Notifio Push Services scaled to 3 concurrent notification workers, but users now receive Passed / Success before Welcome / Started for the same user. Parallel workers broke per-user FIFO — events routed by type land on different workers and finish out of order. Fix get_worker_id() in router.py: route by user_id with consistent hashing (e.g. md5 mod 3), not by event_type. Open a PR when CI passes.