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/DigestWorker.cs

@@ -0,0 +1,69 @@
+using InboxIntel.Application.Abstractions;
+using InboxIntel.Infrastructure.Configuration;
+using InboxIntel.Infrastructure.Persistence;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+
+namespace InboxIntel.Infrastructure.Notifications;
+
+/// <summary>
+/// Hourly tick that sends the inbox digest to opted-in users once their
+/// configured frequency has elapsed. Mirrors <see cref="Sync.GmailSyncWorker"/>.
+/// </summary>
+public class DigestWorker : BackgroundService
+{
+    private readonly IServiceScopeFactory _scopeFactory;
+    private readonly DigestOptions _options;
+    private readonly ILogger<DigestWorker> _logger;
+
+    public DigestWorker(IServiceScopeFactory scopeFactory, IOptions<DigestOptions> options, ILogger<DigestWorker> logger)
+    {
+        _scopeFactory = scopeFactory;
+        _options = options.Value;
+        _logger = logger;
+    }
+
+    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
+    {
+        _logger.LogInformation("DigestWorker started; send hour = {Hour}:00 UTC, frequency = {Days}d", _options.SendHourUtc, _options.FrequencyDays);
+
+        while (!stoppingToken.IsCancellationRequested)
+        {
+            try
+            {
+                if (DateTimeOffset.UtcNow.Hour == _options.SendHourUtc)
+                    await SendDueDigestsAsync(stoppingToken);
+            }
+            catch (Exception ex)
+            {
+                _logger.LogError(ex, "DigestWorker tick failed");
+            }
+
+            await Task.Delay(TimeSpan.FromHours(1), stoppingToken);
+        }
+    }
+
+    private async Task SendDueDigestsAsync(CancellationToken ct)
+    {
+        using var scope = _scopeFactory.CreateScope();
+        var db = scope.ServiceProvider.GetRequiredService<AppDbContext>();
+        var digest = scope.ServiceProvider.GetRequiredService<IDigestService>();
+        var emailSender = scope.ServiceProvider.GetRequiredService<IEmailSender>();
+        if (!emailSender.IsEnabled) return;
+
+        var cutoff = DateTimeOffset.UtcNow.AddDays(-_options.FrequencyDays);
+        var dueUserIds = await db.Users
+            .Where(u => u.DigestEnabled && (u.LastDigestSentUtc == null || u.LastDigestSentUtc <= cutoff))
+            .Select(u => u.Id)
+            .ToListAsync(ct);
+
+        foreach (var userId in dueUserIds)
+        {
+            try { await digest.SendDigestAsync(userId, ct); }
+            catch (Exception ex) { _logger.LogError(ex, "Digest send failed for user {UserId}", userId); }
+        }
+    }
+}