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/Notifications/SmtpEmailSender.cs

@@ -0,0 +1,46 @@
+using InboxIntel.Application.Abstractions;
+using InboxIntel.Infrastructure.Configuration;
+using MailKit.Net.Smtp;
+using MailKit.Security;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+using MimeKit;
+
+namespace InboxIntel.Infrastructure.Notifications;
+
+public class SmtpEmailSender : IEmailSender
+{
+    private readonly SmtpOptions _options;
+    private readonly ILogger<SmtpEmailSender> _logger;
+
+    public SmtpEmailSender(IOptions<SmtpOptions> options, ILogger<SmtpEmailSender> logger)
+    {
+        _options = options.Value;
+        _logger = logger;
+    }
+
+    public bool IsEnabled => _options.Enabled && !string.IsNullOrWhiteSpace(_options.Host);
+
+    public async Task SendAsync(string toAddress, string subject, string htmlBody, CancellationToken ct = default)
+    {
+        if (!IsEnabled)
+        {
+            _logger.LogInformation("SMTP not configured; skipping email \"{Subject}\" to {To}", subject, toAddress);
+            return;
+        }
+
+        var message = new MimeMessage();
+        message.From.Add(new MailboxAddress(_options.FromName, _options.FromAddress));
+        message.To.Add(MailboxAddress.Parse(toAddress));
+        message.Subject = subject;
+        message.Body = new BodyBuilder { HtmlBody = htmlBody }.ToMessageBody();
+
+        using var client = new SmtpClient();
+        var socketOptions = _options.UseSsl ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
+        await client.ConnectAsync(_options.Host, _options.Port, socketOptions, ct);
+        if (!string.IsNullOrEmpty(_options.User))
+            await client.AuthenticateAsync(_options.User, _options.Password, ct);
+        await client.SendAsync(message, ct);
+        await client.DisconnectAsync(true, ct);
+    }
+}