feat: notification digests via SMTP

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>

frontend/src/api/client.js

@@ -148,4 +148,10 @@ export const ExportApi = {
   reportUrl: (format) => `/api/v1/export/report?format=${format}`
 };
 
+export const SettingsApi = {
+  getDigest: () => api.get('/settings/digest').then((r) => r.data),
+  setDigest: (enabled) => api.put('/settings/digest', enabled).then((r) => r.data),
+  sendDigestNow: () => api.post('/settings/digest/send-now'),
+};
+
 export default api;