Email notification channel — delivers a user's notifications to their account
email address via PhoenixKit.Mailer.
Unlike Telegram, email needs no setup: every user has an account email, so
configured?/2 is always true and the Email column is available from the
start. There's no per-user connection or config beyond the routing/cadence the
matrix + aggregation popup write.
The message is core's notification email (PhoenixKit.Email.Content), so
it is sent in the shared layout with the site's branding, can be rewritten
with override files like any other email, and shows in the admin preview.
Its variables are {{subject}} (the notification's title, or the start of
its text), {{text}} and {{url}} (empty when the notification has no
link) — all already rendered in the reader's language.