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.Api/Controllers/SettingsController.cs

@@ -0,0 +1,47 @@
+using InboxIntel.Application.Abstractions;
+using InboxIntel.Infrastructure.Persistence;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.EntityFrameworkCore;
+
+namespace InboxIntel.Api.Controllers;
+
+public record DigestSettingsDto(bool Enabled, DateTimeOffset? LastSentUtc);
+
+/// <summary>User-level preferences. All reads/writes are scoped to the authenticated UserId.</summary>
+public class SettingsController : ApiControllerBase
+{
+    private readonly AppDbContext _db;
+    private readonly IDigestService _digest;
+
+    public SettingsController(AppDbContext db, IDigestService digest)
+    {
+        _db = db;
+        _digest = digest;
+    }
+
+    [HttpGet("digest")]
+    public async Task<IActionResult> GetDigest(CancellationToken ct)
+    {
+        var user = await _db.Users.FirstOrDefaultAsync(u => u.Id == UserId, ct);
+        if (user is null) return NotFound();
+        return Ok(new DigestSettingsDto(user.DigestEnabled, user.LastDigestSentUtc));
+    }
+
+    [HttpPut("digest")]
+    public async Task<IActionResult> SetDigest([FromBody] bool enabled, CancellationToken ct)
+    {
+        var user = await _db.Users.FirstOrDefaultAsync(u => u.Id == UserId, ct);
+        if (user is null) return NotFound();
+        user.DigestEnabled = enabled;
+        await _db.SaveChangesAsync(ct);
+        return Ok(new DigestSettingsDto(user.DigestEnabled, user.LastDigestSentUtc));
+    }
+
+    /// <summary>Sends a digest immediately, for testing/preview.</summary>
+    [HttpPost("digest/send-now")]
+    public async Task<IActionResult> SendNow(CancellationToken ct)
+    {
+        await _digest.SendDigestAsync(UserId, ct);
+        return NoContent();
+    }
+}