feat(bff): owner-scoped self-service withdraw endpoint (S-11c-1, refs #12) (#90)

## What & why

Third sub-slice of **S-11 · Withdrawal (Flow 3)** (#12) — the **owner-scoped BFF withdraw endpoint** (backend). S-11a/b made a withdrawal transition the aggregate and cancel the workflow; this adds the citizen-facing entry point through the BFF, gated to the registration's owner.

- **Domain**: `WithdrawRegistrationCommand` carries the caller's `bsn`; the handler returns a `WithdrawOutcome` and refuses a bsn that doesn't own the registration. Unknown and not-owned are **both 404** (indistinguishable — ownership isn't revealed). `POST /registrations/{id}/withdraw` takes `{bsn}` and maps the outcome (204/404).
- **BFF**: `POST /self-service/registrations/{id}/withdraw` (DigiD-authenticated) forwards the token's `bsn` to the domain and relays 204/404. The BFF authenticates; the domain owner-scopes (an aggregate invariant, not the domain doing auth).
- OpenAPI spec + Angular client regenerated for the new endpoint.
- `run-domain-check.sh` withdrawal step now sends the owner `bsn` (verify-stack).

Refs #12 — the self-service "trek aanvraag in" button + e2e (S-11c-2) closes it.

## Definition of Done

- [x] Linked Gitea issue (#12).
- [x] Failing tests committed before the implementation.
- [x] Implementation makes the tests pass.
- [x] Conventional Commits referencing the issue (`refs #12`).
- [ ] CI green — all Gitea Actions jobs.
- [x] `docker compose up` unaffected.
- [x] No ADR needed (owner-scoping is an aggregate invariant; no boundary change).
- [x] Docs — the user-visible demo note lands with S-11c-2.

## Notes for reviewers

- **Full local gate run before pushing this time** (lessons from #89): `dotnet format --verify-no-changes` clean; `make unit` green — Acl 27, EventSubscriber 19, BFF 30, Acceptance 9, Big 95; `api-client` lint+test green.
- Owner mismatch returns 404 (not 403) so the portal can't be used to probe which references exist.

Reviewed-on: #90

services/bff/Bff.Tests/BffFactory.cs

@@ -82,6 +82,18 @@ internal sealed class FakeDomainClient : IDomainClient
         return Task.FromResult(Result);
     }
 
+    public (string RegistrationId, string Bsn)? Withdrawn { get; private set; }
+
+    /// <summary>Whether the fake domain reports the withdrawal as done (true → 204) or not-found/not-owned
+    /// (false → 404). Tests set this to exercise the relay.</summary>
+    public bool WithdrawSucceeds { get; set; } = true;
+
+    public Task<bool> WithdrawRegistrationAsync(string registrationId, string bsn, CancellationToken ct = default)
+    {
+        Withdrawn = (registrationId, bsn);
+        return Task.FromResult(WithdrawSucceeds);
+    }
+
     public (string RegistrationId, string Besluit)? Decided { get; private set; }
 
     public Task<IReadOnlyList<WerkbakItem>> GetWerkbakAsync(CancellationToken ct = default)

services/bff/Bff.Tests/SelfServiceEndpointTests.cs

@@ -71,5 +71,46 @@ public class SelfServiceEndpointTests
         Assert.Equal("reg-123", body!.RegistrationId);
     }
 
+    private static HttpRequestMessage Withdraw(string? bearer, string id = "reg-123")
+    {
+        var request = new HttpRequestMessage(HttpMethod.Post, $"/self-service/registrations/{id}/withdraw");
+        if (bearer is not null)
+            request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", bearer);
+        return request;
+    }
+
+    [Fact]
+    public async Task Rejects_a_withdrawal_without_a_token()
+    {
+        using var factory = new BffFactory();
+
+        var response = await factory.CreateClient().SendAsync(Withdraw(bearer: null));
+
+        Assert.Equal(HttpStatusCode.Unauthorized, response.StatusCode);
+        Assert.Null(factory.Domain.Withdrawn);
+    }
+
+    [Fact]
+    public async Task Withdraws_the_callers_registration_forwarding_the_id_and_bsn()
+    {
+        using var factory = new BffFactory();
+
+        var response = await factory.CreateClient().SendAsync(Withdraw(TestTokens.Valid("123456782"), "reg-9"));
+
+        Assert.Equal(HttpStatusCode.NoContent, response.StatusCode);
+        Assert.Equal(("reg-9", "123456782"), factory.Domain.Withdrawn);
+    }
+
+    [Fact]
+    public async Task Relays_not_found_when_the_registration_is_unknown_or_not_the_callers()
+    {
+        using var factory = new BffFactory();
+        factory.Domain.WithdrawSucceeds = false;
+
+        var response = await factory.CreateClient().SendAsync(Withdraw(TestTokens.Valid("123456782")));
+
+        Assert.Equal(HttpStatusCode.NotFound, response.StatusCode);
+    }
+
     private sealed record SubmitAcceptedDto(string RegistrationId, string Status);
 }