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.Domain/Entities/User.cs

@@ -26,6 +26,11 @@ public class User : AuditableEntity
 
     public DateTimeOffset? LastLoginUtc { get; set; }
 
+    /// <summary>Whether this user receives periodic inbox digest emails.</summary>
+    public bool DigestEnabled { get; set; } = false;
+
+    public DateTimeOffset? LastDigestSentUtc { get; set; }
+
     public ICollection<Email> Emails { get; set; } = new List<Email>();
     public ICollection<WidgetLayout> WidgetLayouts { get; set; } = new List<WidgetLayout>();
 }