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/Migrations/AppDbContextModelSnapshot.cs

@@ -520,6 +520,9 @@ namespace InboxIntel.Infrastructure.Migrations
                     b.Property<DateTimeOffset>("CreatedAtUtc")
                         .HasColumnType("timestamp with time zone");
 
+                    b.Property<bool>("DigestEnabled")
+                        .HasColumnType("boolean");
+
                     b.Property<string>("DisplayName")
                         .HasColumnType("text");
 
@@ -536,6 +539,9 @@ namespace InboxIntel.Infrastructure.Migrations
                         .HasMaxLength(64)
                         .HasColumnType("character varying(64)");
 
+                    b.Property<DateTimeOffset?>("LastDigestSentUtc")
+                        .HasColumnType("timestamp with time zone");
+
                     b.Property<DateTimeOffset?>("LastLoginUtc")
                         .HasColumnType("timestamp with time zone");