Adds the bin/worker-notifications entrypoint, Dockerfile chmod, and
docker-compose service definitions for the new Notifications worker
without disturbing the existing Mails worker, and re-adds the Mails
queue/class constants on Event so the legacy callers still resolve.
The full mail->notification swap (caller migration, Mails removal,
worker rename) will land as a follow-up.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
