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.Api/Program.cs

@@ -26,6 +26,7 @@ builder.Services.AddHttpClient<IAclClient, AclHttpClient>();
 builder.Services.AddScoped<SubmitRegistration>();
 builder.Services.AddScoped<ApproveRegistration>();
 builder.Services.AddScoped<BeoordeelRegistratie>();
+builder.Services.AddScoped<WithdrawRegistration>();
 builder.Services.AddScoped<Werkbak>();
 builder.Services.AddScoped<OpenZaakWorker>();
 builder.Services.AddScoped<OpenZaakJobProcessor>();
@@ -74,6 +75,19 @@ app.MapPost("/registrations/{id}/decide", async (string id, DecideRequest body,
     return Results.NoContent();
 });
 
+// Withdraw a registration (S-11): the zorgprofessional pulls their own still-open submission back,
+// advancing it to INGETROKKEN. Idempotent. The BFF reaches this behind a digid token, owner-scoped
+// to the caller's bsn (S-11c); the domain trusts its callers (§8.3). Cancelling the running Flowable
+// process is a later sub-slice (S-11b).
+app.MapPost("/registrations/{id}/withdraw", async (string id, WithdrawRegistration withdraw, CancellationToken ct) =>
+{
+    if (!Guid.TryParse(id, out var guid))
+        return Results.NotFound();
+
+    await withdraw.HandleAsync(new WithdrawRegistrationCommand(new RegistrationId(guid)), ct);
+    return Results.NoContent();
+});
+
 // The behandelaar's werkbak (S-12): the registrations awaiting beoordeling, read from the open
 // Beoordelen user tasks (§8.2) and enriched with bsn + status. The BFF proxies this behind
 // medewerker-realm + behandelaar-role authorization; the domain trusts its callers (§8.3).