feat(bff): POST /self-service/registrations/{id}/withdraw, owner-scoped (refs #12)

The domain withdraw command carries the caller's bsn and owner-scopes the aggregate; unknown or

not-owned is 404 (indistinguishable). The BFF forwards the DigiD token's bsn and relays 204/404.

Regenerate the OpenAPI spec + Angular client for the new endpoint.

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

tests/acceptance/Support/BffAcceptanceHost.cs

@@ -69,6 +69,9 @@ public sealed class CapturingDomainClient : IDomainClient
         return Task.FromResult(new SubmitAccepted("reg-acc-1", "Ingediend"));
     }
 
+    public Task<bool> WithdrawRegistrationAsync(string registrationId, string bsn, CancellationToken ct = default)
+        => Task.FromResult(true);
+
     public Task<IReadOnlyList<WerkbakItem>> GetWerkbakAsync(CancellationToken ct = default)
         => Task.FromResult<IReadOnlyList<WerkbakItem>>([]);