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.Infrastructure/Configuration/Options.cs

@@ -48,3 +48,25 @@ public class AiOptions
     public string OpenAiApiKey { get; set; } = string.Empty;
     public string OpenAiModel { get; set; } = "gpt-4o-mini";
 }
+
+public class SmtpOptions
+{
+    public const string SectionName = "Smtp";
+    public bool Enabled { get; set; } = false;
+    public string Host { get; set; } = string.Empty;
+    public int Port { get; set; } = 587;
+    public bool UseSsl { get; set; } = true;
+    public string User { get; set; } = string.Empty;
+    public string Password { get; set; } = string.Empty;
+    public string FromAddress { get; set; } = string.Empty;
+    public string FromName { get; set; } = "InboxIntel";
+}
+
+public class DigestOptions
+{
+    public const string SectionName = "Digest";
+    /// <summary>How often a digest is sent to each opted-in user.</summary>
+    public int FrequencyDays { get; set; } = 7;
+    /// <summary>Hour (UTC) the background worker checks for due digests.</summary>
+    public int SendHourUtc { get; set; } = 8;
+}