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/ApplicationStore.cs

@@ -26,6 +26,13 @@ public sealed class Aanvraag
   public DateTimeOffset CreatedAt { get; init; }
   public DateTimeOffset UpdatedAt { get; set; }
   public DateTimeOffset? SubmittedAt { get; set; }
+
+  /// <summary>The OpenZaak zaak's URL, set once CreateZaak (WP-50) registers one — null under
+  /// the local source. Persisted so later steps (WP-51's document→zaak link) can find it
+  /// without a network round-trip; IZaakSource.CreateZaak itself doesn't write here (the
+  /// endpoint does, via <see cref="ApplicationStore.SetZaakUrl"/>) to keep the seam's write
+  /// surface at "return data", not "reach into another store".</summary>
+  public string? ZaakUrl { get; set; }
 }
 
 /// <summary>
@@ -172,4 +179,18 @@ public static class ApplicationStore
       return a;
     }
   }
+
+  /// <summary>Persist the zaak URL CreateZaak (WP-50) registered for this aanvraag. No-op if
+  /// the aanvraag is gone (shouldn't happen — this runs right after Submit found it).</summary>
+  public static void SetZaakUrl(string id, string zaakUrl)
+  {
+    lock (_gate)
+    {
+      using var db = Db.Create();
+      var a = db.Applications.Find(id);
+      if (a is null) return;
+      a.ZaakUrl = zaakUrl;
+      db.SaveChanges();
+    }
+  }
 }