feat(domain): withdrawal — INGETROKKEN transition + endpoint (S-11a, refs #12) (#88)

## What & why

First sub-slice of **S-11 · Withdrawal (Flow 3)** (#12). A zorgprofessional can withdraw a still-open registration ("trek aanvraag in"); this sub-slice delivers the **domain transition + endpoint**, mirroring how S-12a shipped the beoordeling decision model on its own (#82).

- `RegistrationStatus.Ingetrokken` (terminal).
- `Registration.Withdraw()` — allowed from INGEDIEND or IN_BEHANDELING, needs no zaak, idempotent, and rejected once the registration has been decided (INGESCHREVEN/AFGEWEZEN).
- `WithdrawRegistration` application handler (load → withdraw → persist; repeated withdrawal is a no-op).
- `POST /registrations/{id}/withdraw` on the domain API.

Demoable: `POST /registrations/{id}/withdraw` → `GET /registrations/{id}` shows `INGETROKKEN`.

Refs #12 (not closing — see below).

## Scope / follow-ups

S-11 is bigger than one slice, so it is split (CLAUDE.md §13), like S-12 was:
- **S-11a (this PR)** — domain withdrawal transition + endpoint.
- **S-11b** — cancel the running Flowable process via a BPMN message event, so a withdrawn case leaves the behandelaar's werkbak.
- **S-11c** — owner-scoped BFF self-service withdraw endpoint + "trek aanvraag in" button + e2e.

Cancelling the Flowable process is deliberately deferred (documented in `WithdrawRegistration`), exactly as the beoordeling's rejection deferred its zaak propagation. #12 stays open until S-11c.

## Definition of Done

- [x] Linked Gitea issue (#12).
- [x] Failing test committed before the implementation.
- [x] Implementation makes the test pass.
- [x] Conventional Commits referencing the issue (`refs #12`).
- [ ] CI green — all Gitea Actions jobs.
- [x] `docker compose up` unaffected (no infra/contract change).
- [x] Docs — none needed for this backend sub-slice; the user-visible demo note lands with S-11c.
- [x] No ADR needed — mirrors existing aggregate/handler/endpoint patterns; no boundary change.

## Notes for reviewers

- Verified locally: `Big.Tests` 89/89 pass; `Big.Api` builds clean.
- The domain trusts its callers (§8.3); owner-scoping by the caller's bsn is enforced at the BFF in S-11c.

Reviewed-on: #88

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).

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);
+    }
+}

services/domain/Big.Domain/Registration.cs

@@ -111,7 +111,23 @@ public sealed class Registration
         Status = RegistrationStatus.Afgewezen;
     }
 
-    // A decision is only valid while the registration is still open (INGEDIEND or IN_BEHANDELING).
+    /// <summary>
+    /// Withdraw the registration — the zorgprofessional pulls their own submission back (S-11). Allowed
+    /// while it is still open (INGEDIEND or IN_BEHANDELING) and needs no zaak; a registration that has
+    /// already been decided (INGESCHREVEN/AFGEWEZEN) can no longer be withdrawn. Re-withdrawing one
+    /// already <see cref="RegistrationStatus.Ingetrokken"/> is a no-op.
+    /// </summary>
+    public void Withdraw()
+    {
+        if (Status == RegistrationStatus.Ingetrokken)
+            return;
+
+        RequireOpenForDecision(nameof(Withdraw));
+        Status = RegistrationStatus.Ingetrokken;
+    }
+
+    // A decision (or withdrawal) is only valid while the registration is still open (INGEDIEND or
+    // IN_BEHANDELING).
     private void RequireOpenForDecision(string decision)
     {
         if (Status is not (RegistrationStatus.Ingediend or RegistrationStatus.InBehandeling))

services/domain/Big.Domain/RegistrationStatus.cs

@@ -3,7 +3,8 @@ namespace Big.Domain;
 /// <summary>The lifecycle states a <see cref="Registration"/> moves through. Submission starts in
 /// <see cref="Ingediend"/>; a behandelaar takes it <see cref="InBehandeling"/> and decides it into one
 /// of the terminal states <see cref="Ingeschreven"/> (approved) or <see cref="Afgewezen"/> (rejected).
-/// Withdrawal and herregistratie states arrive in their own slices (S-11+).</summary>
+/// A zorgprofessional can withdraw a still-open registration into <see cref="Ingetrokken"/> (S-11).
+/// The herregistratie state arrives in its own slice.</summary>
 public enum RegistrationStatus
 {
     /// <summary>Submitted by the zorgprofessional; the registratie process has been started.</summary>
@@ -17,4 +18,7 @@ public enum RegistrationStatus
 
     /// <summary>Rejected by the behandelaar. Terminal.</summary>
     Afgewezen,
+
+    /// <summary>Withdrawn by the zorgprofessional before a decision (S-11). Terminal.</summary>
+    Ingetrokken,
 }

services/domain/Big.Tests/RegistrationTests.cs

@@ -217,4 +217,72 @@ public class RegistrationTests
         Assert.Contains("Afgewezen", rejectEx.Message);
         Assert.Equal(RegistrationStatus.Afgewezen, registration.Status);
     }
+
+    [Fact]
+    public void Withdrawing_an_ingediend_registration_sets_it_ingetrokken()
+    {
+        var registration = Registration.Submit("123456782");
+
+        registration.Withdraw();
+
+        Assert.Equal(RegistrationStatus.Ingetrokken, registration.Status);
+    }
+
+    [Fact]
+    public void Withdrawing_an_in_behandeling_registration_sets_it_ingetrokken()
+    {
+        // A citizen can still pull a registration back while a behandelaar has it in behandeling.
+        var registration = Registration.Submit("123456782");
+        registration.TakeIntoBehandeling();
+
+        registration.Withdraw();
+
+        Assert.Equal(RegistrationStatus.Ingetrokken, registration.Status);
+    }
+
+    [Fact]
+    public void Withdrawing_needs_no_zaak()
+    {
+        // Withdrawal is the citizen's own action and does not depend on the zaak having been opened.
+        var registration = Registration.Submit("123456782");
+
+        registration.Withdraw();
+
+        Assert.Equal(RegistrationStatus.Ingetrokken, registration.Status);
+        Assert.Null(registration.ZaakUrl);
+    }
+
+    [Fact]
+    public void Re_withdrawing_an_already_ingetrokken_registration_is_idempotent()
+    {
+        var registration = Registration.Submit("123456782");
+        registration.Withdraw();
+
+        registration.Withdraw();
+
+        Assert.Equal(RegistrationStatus.Ingetrokken, registration.Status);
+    }
+
+    [Fact]
+    public void Withdrawing_an_approved_registration_is_rejected()
+    {
+        var registration = Registration.Submit("123456782");
+        registration.AttachZaak(new Uri("http://openzaak/zaken/api/v1/zaken/abc"));
+        registration.Approve();
+
+        var ex = Assert.Throws<InvalidOperationException>(() => registration.Withdraw());
+        Assert.Contains("only an INGEDIEND", ex.Message);
+        Assert.Equal(RegistrationStatus.Ingeschreven, registration.Status);
+    }
+
+    [Fact]
+    public void Withdrawing_a_rejected_registration_is_rejected()
+    {
+        var registration = Registration.Submit("123456782");
+        registration.Reject();
+
+        var ex = Assert.Throws<InvalidOperationException>(() => registration.Withdraw());
+        Assert.Contains("only an INGEDIEND", ex.Message);
+        Assert.Equal(RegistrationStatus.Afgewezen, registration.Status);
+    }
 }

services/domain/Big.Tests/WithdrawRegistrationTests.cs

@@ -0,0 +1,60 @@
+using Big.Application;
+using Big.Domain;
+
+namespace Big.Tests;
+
+public class WithdrawRegistrationTests
+{
+    [Fact]
+    public async Task Withdrawing_marks_the_registration_ingetrokken_and_persists_it()
+    {
+        var store = new FakeRegistrationStore();
+        var registration = Registration.Submit("123456782");
+        store.Seed(registration);
+        var handler = new WithdrawRegistration(store);
+
+        await handler.HandleAsync(new WithdrawRegistrationCommand(registration.Id));
+
+        var saved = await store.GetAsync(registration.Id);
+        Assert.Equal(RegistrationStatus.Ingetrokken, saved!.Status);
+        Assert.Equal(1, store.SaveCount);
+    }
+
+    [Fact]
+    public async Task Rejects_a_null_command_without_touching_the_store()
+    {
+        var store = new FakeRegistrationStore();
+        var handler = new WithdrawRegistration(store);
+
+        await Assert.ThrowsAsync<ArgumentNullException>(() => handler.HandleAsync(null!));
+        Assert.Equal(0, store.SaveCount);
+    }
+
+    [Fact]
+    public async Task Withdrawing_an_unknown_registration_throws()
+    {
+        var store = new FakeRegistrationStore();
+        var handler = new WithdrawRegistration(store);
+
+        var ex = await Assert.ThrowsAsync<InvalidOperationException>(
+            () => handler.HandleAsync(new WithdrawRegistrationCommand(RegistrationId.New())));
+        Assert.Contains("No registration", ex.Message);
+        Assert.Equal(0, store.SaveCount);
+    }
+
+    [Fact]
+    public async Task Re_withdrawing_an_already_ingetrokken_registration_is_idempotent()
+    {
+        var store = new FakeRegistrationStore();
+        var registration = Registration.Submit("123456782");
+        store.Seed(registration);
+        var handler = new WithdrawRegistration(store);
+
+        await handler.HandleAsync(new WithdrawRegistrationCommand(registration.Id));
+        await handler.HandleAsync(new WithdrawRegistrationCommand(registration.Id));
+
+        // The second withdrawal is a no-op: the aggregate is not persisted again.
+        Assert.Equal(1, store.SaveCount);
+        Assert.Equal(RegistrationStatus.Ingetrokken, (await store.GetAsync(registration.Id))!.Status);
+    }
+}