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;

frontend/src/components/DigestToggle.jsx

@@ -0,0 +1,28 @@
+import { useEffect, useState } from 'react';
+import { SettingsApi } from '../api/client.js';
+
+export default function DigestToggle() {
+  const [enabled, setEnabled] = useState(null);
+
+  useEffect(() => {
+    SettingsApi.getDigest().then((d) => setEnabled(d.enabled)).catch(() => setEnabled(false));
+  }, []);
+
+  const toggle = async () => {
+    const next = !enabled;
+    setEnabled(next);
+    try { await SettingsApi.setDigest(next); } catch { setEnabled(!next); }
+  };
+
+  if (enabled === null) return null;
+
+  return (
+    <button
+      className="ghost digest-toggle"
+      title={enabled ? 'Weekly digest emails: on' : 'Weekly digest emails: off'}
+      onClick={toggle}
+    >
+      {enabled ? '🔔' : '🔕'}
+    </button>
+  );
+}

frontend/src/components/Layout.jsx

@@ -4,6 +4,7 @@ import { AuthApi, SyncApi, AnalyticsApi } from '../api/client.js';
 import Logo from './Logo.jsx';
 import DevBanner from './DevBanner.jsx';
 import SyncStatus from './SyncStatus.jsx';
+import DigestToggle from './DigestToggle.jsx';
 import useSavedSearches from '../hooks/useSavedSearches.js';
 
 // ── Folder definitions ────────────────────────────────────────────────────────
@@ -255,6 +256,7 @@ export default function Layout() {
         </form>
         <div className="spacer" />
         <SyncStatus />
+        <DigestToggle />
         <button onClick={startSync}>Sync now</button>
         <span className="user">{user?.email}</span>
         <button className="ghost" onClick={logout}>Log out</button>

frontend/src/styles.css

@@ -412,3 +412,5 @@ input, select { background: var(--panel-2); border: 1px solid #2c3550; color: va
 .saved-search-save-btn:hover { color: var(--text); border-color: var(--accent); }
 
 .email-row--focused { outline: 1px solid var(--accent); outline-offset: -1px; }
+
+.digest-toggle { font-size: 16px; padding: 4px 8px; }