feat(zgw): OpenZaak Documenten (DRC) upload + zaak link (WP-51)

Extends the OpenZaak seam with IDocumentSource, sibling of IZaakSource
(WP-49/50): an upload always lands locally first (DocumentStore stays
the record of truth for preview/download/audit) and, when
Zgw:Enabled=true, is also registered as a DRC enkelvoudiginformatie-
object; once a zaak exists (IZaakSource.CreateZaak now also returns
its ZaakUrl), submit links each document to it via zaakinformatie-
object. FE upload/list DTOs are unchanged.

- ZgwOptions gains DrcBaseUrl + a category->informatieobjecttype URL
  map (the document analogue of ZaaktypeUrls).
- LocalDocumentSource is the same DocumentStore.Add/Link calls the
  endpoints used to make inline — zero behaviour change offline.
- OpenZaakDocumentSource POSTs the eio then the zaak link, persisting
  the DRC url (DocumentStore.SetDrcUrl) so linking doesn't re-upload.
- Factored the GET/POST-with-bearer-JWT plumbing shared with
  OpenZaakZaakSource into ZgwHttpClient; shared the stub handler
  between the two source test classes as ZgwStubHandler.

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

backend/tests/BigRegister.Tests/ZgwStubHandler.cs

@@ -0,0 +1,32 @@
+using System.Net;
+using System.Text;
+
+namespace BigRegister.Tests;
+
+/// <summary>
+/// Stub HttpMessageHandler shared by the ZGW source tests (no live server, no mocking
+/// library) — keyed purely by request URL (method-agnostic, since no test scenario reuses a
+/// URL across GET/POST). Records every request's url/body/auth-scheme for assertion.
+/// Factored out of OpenZaakZaakSourceTests once OpenZaakDocumentSourceTests needed the
+/// identical stub.
+/// </summary>
+internal sealed class ZgwStubHandler(Func<string, string> respond) : HttpMessageHandler
+{
+  public List<string> Requests { get; } = new();
+  public List<string?> AuthSchemes { get; } = new();
+  public List<string> Bodies { get; } = new();
+
+  public string BodyOf(string url) => Bodies[Requests.LastIndexOf(url)];
+
+  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"),
+    });
+  }
+}