feat(domain): withdraw a registration → INGETROKKEN via POST /registrations/{id}/withdraw (refs #12)

Add the Ingetrokken terminal status, Registration.Withdraw() (open-only, idempotent), the

WithdrawRegistration handler, and the domain endpoint. Cancelling the running Flowable process

(so the case leaves the werkbak) and the owner-scoped BFF/self-service action are follow-up

sub-slices (S-11b/S-11c).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

services/domain/Big.Application/WithdrawRegistration.cs

@@ -0,0 +1,32 @@
+using Big.Domain;
+
+namespace Big.Application;
+
+/// <summary>A zorgprofessional's request to withdraw their own registration ("trek aanvraag in").</summary>
+public sealed record WithdrawRegistrationCommand(RegistrationId RegistrationId);
+
+/// <summary>
+/// The withdrawal use case (S-11): a zorgprofessional pulls a still-open registration back. It
+/// advances the aggregate to INGETROKKEN and persists it. Idempotent — a repeated or redelivered
+/// withdrawal of an already-withdrawn registration is a no-op (the aggregate is not persisted again).
+/// Cancelling the running Flowable process (so the case leaves the behandelaar's werkbak) is a later
+/// sub-slice (S-11b, via a BPMN message event); this sub-slice owns the domain transition only —
+/// mirroring how the beoordeling's rejection deferred its zaak propagation.
+/// </summary>
+public sealed class WithdrawRegistration(IRegistrationStore store)
+{
+    public async Task HandleAsync(WithdrawRegistrationCommand command, CancellationToken ct = default)
+    {
+        ArgumentNullException.ThrowIfNull(command);
+
+        var registration = await store.GetAsync(command.RegistrationId, ct)
+            ?? throw new InvalidOperationException($"No registration {command.RegistrationId} to withdraw.");
+
+        // A repeated withdrawal is a no-op: don't persist the already-withdrawn aggregate again.
+        if (registration.Status == RegistrationStatus.Ingetrokken)
+            return;
+
+        registration.Withdraw();
+        await store.SaveAsync(registration, ct);
+    }
+}