feat(domain): Quartz cron job fires the herregistratie sweep; expose deadline on read (refs #18)

HerregistratieReminderJob (Quartz IJob) fires HerregistratieReminderSweep on a
daily cron wired in Big.Api (overridable via Quartz__Cron), and logs how many
reminders went out. Quartz.NET is used for this time-triggered fleet sweep,
distinct from the queue-draining pumps (ADR-0022). GET /registrations/{id} now
returns herregistratieVoor + herregistratieReminderVerstuurd. The scheduling
shell is excluded from mutation, mirroring the pumps.

refs #18

tests/acceptance/Support/InMemoryDomainPorts.cs

@@ -221,4 +221,9 @@ public sealed class InMemoryRegistrationStore : IRegistrationStore
     public Task<Registration?> FindOpenByBsnAsync(string bsn, CancellationToken ct = default)
         => Task.FromResult(_byId.Values.FirstOrDefault(r =>
             r.Bsn == bsn && r.Status is RegistrationStatus.Ingediend or RegistrationStatus.InBehandeling));
+
+    public Task<IReadOnlyList<Registration>> FindDueForHerregistratieReminderAsync(
+        DateTimeOffset asOf, CancellationToken ct = default)
+        => Task.FromResult<IReadOnlyList<Registration>>(
+            _byId.Values.Where(r => r.HerregistratieReminderDue(asOf)).ToList());
 }