Adds email as a notification channel built on the shared SMTP core
(mail.Sender). Email mode is a per-account preference (off, daily
digest, or per-episode) stored in notification_email_prefs; delivery is
an account-watermark sweep over notification_deliveries that dedupes
cross-profile duplicates, advancing the watermark only after a
successful send. Admin controls cover the channel kill switch, the
per-episode allowance (off coerces those accounts to the digest),
digest hour, and an external URL for deep links inside emails.
Availability is advertised through /notifications/capability and the
user settings page gains an Email section for opt-in.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
