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>
Imports the notification system design folder (architecture overview,
release-events/inbox foundation, APNs/FCM relay specs, outbound webhooks)
and adds the Web Push spec (05, implemented in this branch), the shared
outbound-email architecture note, and the v1.5 roadmap (06) covering the
remaining work after APNs/FCM were deferred to v2.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
