Artifact: PR Description — Migrate Notification Worker to Kafka

PR description with explicit review guidance for a large diff -- exercises co-09.

What: migrates NotificationWorker from the legacy in-process queue consumer to a Kafka consumer, per ADR-0005.

Why: enables message replay after worker bugs instead of manual data reconciliation.

How verified: full existing Notification Worker test suite passes against a local Kafka instance; ran a replay drill in staging (killed the consumer mid-batch, restarted, confirmed every message the old queue would have delivered is still delivered, in order, under the new Kafka consumer).

Where to review first: notification_worker/consumer.py's handle_rebalance() method -- everything else in this diff (import changes, config wiring, thirteen other files) is mechanical. handle_rebalance() is the one place a consumer-group rebalance could replay already-processed messages if the offset-commit logic has a bug; get this one function right and the rest of the diff is low-risk by comparison.