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/IZaakSource.cs

@@ -22,11 +22,13 @@ public interface IZaakSource
   /// <summary>
   /// Register a just-submitted <paramref name="aanvraag"/> as a zaak (WP-50). The aanvraag is
   /// already persisted locally (<c>ApplicationStore.Submit</c> already ran) — this is the
-  /// integration side-effect, and its return value is what the submit endpoint hands back to
-  /// the FE (ADR-0001: route the create through the existing submit response DTO, don't add a
-  /// second one). The local source is a pure passthrough of the already-computed local
-  /// reference/status; the OpenZaak source creates a Zaak (+ status + rol) and maps the result
-  /// back into the same shape.
+  /// integration side-effect, and (Referentie, Status) is what the submit endpoint hands back
+  /// to the FE (ADR-0001: route the create through the existing submit response DTO, don't add
+  /// a second one). The local source is a pure passthrough of the already-computed local
+  /// reference/status (ZaakUrl null — nothing to persist); the OpenZaak source creates a Zaak
+  /// (+ status + rol) and maps the result back into the same shape, returning the zaak's URL
+  /// so the endpoint can persist it (<see cref="ApplicationStore.SetZaakUrl"/>, WP-51 needs it
+  /// to later link documents to this zaak).
   /// </summary>
-  (string Referentie, AanvraagStatusDto Status) CreateZaak(Aanvraag aanvraag, DateTimeOffset now);
+  (string Referentie, AanvraagStatusDto Status, string? ZaakUrl) CreateZaak(Aanvraag aanvraag, DateTimeOffset now);
 }