feat(zgw): OpenZaak create-zaak, first write slice (WP-50)

Extends the IZaakSource seam (WP-49, read-only) with CreateZaak: submitting
an aanvraag now also registers a Zaak + Status + Rol in OpenZaak when
Zgw:Enabled=true, routed through the existing /applications/{id}/submit
endpoint with the FE response DTO unchanged (ADR-0001/ADR-0005 — the
endpoint never branches on the config flag itself, DI already picked the
implementation).

- ZgwOptions gains a Type→zaaktype-URL map + the two RSINs a Zaak needs.
- LocalZaakSource.CreateZaak is a pure passthrough of what the endpoint
  already computes locally (zero behaviour change for the offline default).
- OpenZaakZaakSource.CreateZaak POSTs the zaak (identificatie = the same
  local reference, so both stay in sync), resolves + POSTs the initial
  status and the initiator rol (BSN) via Catalogi lookups, and maps the
  result back into the submit response.
- Marked ponytail shortcuts: first-statustype/roltype-Catalogi-returns
  (no per-type config) and no compensating transaction on partial failure
  — both fine for a first slice against a demo backend.

Verified: full `npm run ci` green, zero api-client drift, 144/144 backend
tests (142 existing + 2 new stub-handler tests asserting the POST bodies
+ type→zaaktype mapping per the acceptance criteria).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/tests/BigRegister.Tests/OpenZaakZaakSourceTests.cs

@@ -1,5 +1,6 @@
 using System.Net;
 using System.Text;
+using BigRegister.Api.Data;
 using BigRegister.Api.Zgw;
 
 namespace BigRegister.Tests;
@@ -59,16 +60,98 @@ public class OpenZaakZaakSourceTests
     Assert.All(handler.AuthSchemes, s => Assert.Equal("Bearer", s));
   }
 
+  [Fact]
+  public void CreateZaak_posts_zaak_status_and_rol_and_maps_the_result_back()
+  {
+    const string zaaktypeUrl = $"{ZtBase}/zaaktypen/zt-registratie";
+    var handler = new StubHandler(url => url switch
+    {
+      _ when url == $"{ZrcBase}/zaken" => $$"""
+        { "url": "{{ZrcBase}}/zaken/uuid-new", "identificatie": "BIG-2026-000123",
+          "zaaktype": "{{zaaktypeUrl}}", "startdatum": "2026-07-28",
+          "einddatum": null, "registratiedatum": "2026-07-28" }
+        """,
+      _ when url.StartsWith($"{ZtBase}/statustypen") => """
+        { "count": 1, "next": null,
+          "results": [ { "url": "https://oz.example/catalogi/api/v1/statustypen/st-1", "volgnummer": 1 } ] }
+        """,
+      _ when url.StartsWith($"{ZtBase}/roltypen") => """
+        { "count": 1, "next": null,
+          "results": [ { "url": "https://oz.example/catalogi/api/v1/roltypen/rt-initiator" } ] }
+        """,
+      _ when url == $"{ZrcBase}/statussen" => "{}",
+      _ when url == $"{ZrcBase}/rollen" => "{}",
+      _ => throw new InvalidOperationException($"unexpected ZGW call {url}"),
+    });
+
+    var options = new ZgwOptions
+    {
+      ZrcBaseUrl = ZrcBase,
+      ZtcBaseUrl = ZtBase,
+      ClientId = "c",
+      Secret = "s",
+      Bronorganisatie = "123443210",
+      VerantwoordelijkeOrganisatie = "123443210",
+      ZaaktypeUrls = new() { ["registratie"] = zaaktypeUrl },
+    };
+    var source = new OpenZaakZaakSource(new HttpClient(handler), new ZgwTokenProvider(options), options);
+    var aanvraag = new Aanvraag
+    {
+      Id = "a1",
+      Type = "registratie",
+      Owner = "111222333",
+      Referentie = "BIG-2026-000123",
+    };
+
+    var (referentie, status) = source.CreateZaak(aanvraag, new DateTimeOffset(2026, 7, 28, 12, 0, 0, TimeSpan.Zero));
+
+    Assert.Equal("BIG-2026-000123", referentie);
+    Assert.Equal("InBehandeling", status.Tag);
+    Assert.Equal("BIG-2026-000123", status.Referentie);
+
+    string BodyOf(string url) => handler.Bodies[handler.Requests.LastIndexOf(url)];
+
+    // Zaak: mapped zaaktype + configured RSINs + the local reference as identificatie.
+    var zaakBody = BodyOf($"{ZrcBase}/zaken");
+    Assert.Contains(zaaktypeUrl, zaakBody);
+    Assert.Contains("123443210", zaakBody);
+    Assert.Contains("BIG-2026-000123", zaakBody);
+
+    // Status: points at the created zaak's URL and the resolved statustype.
+    var statusBody = BodyOf($"{ZrcBase}/statussen");
+    Assert.Contains($"{ZrcBase}/zaken/uuid-new", statusBody);
+    Assert.Contains("statustypen/st-1", statusBody);
+
+    // Rol: points at the created zaak, the resolved initiator roltype, and the BSN.
+    var rolBody = BodyOf($"{ZrcBase}/rollen");
+    Assert.Contains($"{ZrcBase}/zaken/uuid-new", rolBody);
+    Assert.Contains("roltypen/rt-initiator", rolBody);
+    Assert.Contains("111222333", rolBody);
+  }
+
+  [Fact]
+  public void CreateZaak_throws_when_the_aanvraag_type_has_no_configured_zaaktype()
+  {
+    var options = new ZgwOptions { ZrcBaseUrl = ZrcBase, ZtcBaseUrl = ZtBase, ClientId = "c", Secret = "s" };
+    var handler = new StubHandler(url => throw new InvalidOperationException($"no HTTP call expected, got {url}"));
+    var source = new OpenZaakZaakSource(new HttpClient(handler), new ZgwTokenProvider(options), options);
+    var aanvraag = new Aanvraag { Id = "a1", Type = "unknown-type", Owner = "111222333", Referentie = "BIG-2026-000123" };
+
+    Assert.Throws<InvalidOperationException>(() => source.CreateZaak(aanvraag, DateTimeOffset.UtcNow));
+  }
+
   private sealed class StubHandler(Func<string, string> respond) : HttpMessageHandler
   {
     public List<string> Requests { get; } = new();
     public List<string?> AuthSchemes { get; } = new();
+    public List<string> Bodies { get; } = new();
 
     protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
     {
       var url = request.RequestUri!.ToString();
       Requests.Add(url);
       AuthSchemes.Add(request.Headers.Authorization?.Scheme);
+      Bodies.Add(request.Content?.ReadAsStringAsync(cancellationToken).GetAwaiter().GetResult() ?? "");
       return Task.FromResult(new HttpResponseMessage(HttpStatusCode.OK)
       {
         Content = new StringContent(respond(url), Encoding.UTF8, "application/json"),