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.Application/Abstractions/IServices.cs

@@ -73,3 +73,16 @@ public interface IExportService
 {
     Task<(byte[] Content, string ContentType, string FileName)> ExportReportAsync(Guid userId, ExportFormat format, CancellationToken ct = default);
 }
+
+/// <summary>SMTP-backed email delivery, used for digest notifications. No-ops if SMTP is not configured.</summary>
+public interface IEmailSender
+{
+    bool IsEnabled { get; }
+    Task SendAsync(string toAddress, string subject, string htmlBody, CancellationToken ct = default);
+}
+
+/// <summary>Builds and sends the periodic inbox digest for opted-in users.</summary>
+public interface IDigestService
+{
+    Task SendDigestAsync(Guid userId, CancellationToken ct = default);
+}