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>

src/InboxIntel.Api/appsettings.json

@@ -37,6 +37,20 @@
     "OpenAiApiKey": "",
     "OpenAiModel": "gpt-4o-mini"
   },
+  "Smtp": {
+    "Enabled": false,
+    "Host": "",
+    "Port": 587,
+    "UseSsl": true,
+    "User": "",
+    "Password": "",
+    "FromAddress": "",
+    "FromName": "InboxIntel"
+  },
+  "Digest": {
+    "FrequencyDays": 7,
+    "SendHourUtc": 8
+  },
   "Cors": {
     "Origins": [ "http://localhost:5173" ]
   },