Adds a periodic inbox-summary email per opted-in user. SMTP is
configured via appsettings (Smtp section); the digest is built from
existing analytics (health score, top senders, recommendations) and
sent by a new hourly DigestWorker, mirroring the GmailSyncWorker
pattern. Frequency and send hour are configurable (Digest section).
Backend: IEmailSender (SmtpEmailSender, MailKit), IDigestService,
DigestWorker, new User.DigestEnabled/LastDigestSentUtc fields +
migration, SettingsController for the per-user toggle and a
send-now test endpoint, all scoped to the authenticated UserId.
Frontend: SettingsApi + a bell/no-bell toggle button in the topbar.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
cesnimda