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/src/BigRegister.Api/Data/DocumentStore.cs

@@ -11,6 +11,12 @@ public sealed record StoredDocument(
     string FileName, long SizeBytes, string ContentType, byte[] Content, string Owner, DateTimeOffset UploadedAt)
 {
   public bool Linked { get; set; }
+
+  /// <summary>The OpenZaak DRC enkelvoudiginformatieobject's URL, set once Upload (WP-51)
+  /// registers one — null under the local source. Persisted so the later zaak-link step can
+  /// find it without re-uploading; not part of the positional constructor, same reasoning as
+  /// <see cref="Linked"/> (every existing `new StoredDocument(...)` call site keeps working).</summary>
+  public string? DrcUrl { get; set; }
 }
 
 /// <summary>Id is EF Core's auto-increment key — not part of the positional
@@ -69,6 +75,19 @@ public static class DocumentStore
     }
   }
 
+  /// <summary>Persist the DRC url an OpenZaak upload (WP-51) registered for a document.</summary>
+  public static void SetDrcUrl(string documentId, string drcUrl)
+  {
+    lock (_gate)
+    {
+      using var db = Db.Create();
+      var d = db.Documents.Find(documentId);
+      if (d is null) return;
+      d.DrcUrl = drcUrl;
+      db.SaveChanges();
+    }
+  }
+
   /// Mark digital documents as linked to a finalised submission (blocks user delete).
   public static void Link(IEnumerable<string> documentIds)
   {