feat(acl): store a diploma in the Documenten API and relate it to the zaak (refs #103)

IZaakGateway.StoreDocumentAsync creates an enkelvoudiginformatieobject (base64
inhoud, buffered non-chunked body, no CRS) and relates it via zaakinformatieobjecten;
AclService.StoreDiplomaAsync default-fills the ZGW document fields (informatieobjecttype,
bronorganisatie, taal nld, creatiedatum); POST /documenten exposes it. Adds the
InformatieobjecttypeUrl default (wired in a following infra commit).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

services/acl/Acl.Application/IZaakGateway.cs

@@ -16,4 +16,11 @@ public interface IZaakGateway
     /// <summary>Read the zaak's <c>identificatie</c> — the public-safe reference the register shows.
     /// The Event Subscriber calls this through the ACL rather than reading ZGW itself (§8.1, #78).</summary>
     Task<string> GetZaakIdentificatieAsync(Uri zaakUrl, CancellationToken ct = default);
+
+    /// <summary>
+    /// Store a diploma document (S-10b): create an <c>enkelvoudiginformatieobject</c> in the ZGW
+    /// Documenten API and relate it to the zaak via a <c>zaakinformatieobject</c>. Returns the URL of
+    /// the created informatieobject.
+    /// </summary>
+    Task<Uri> StoreDocumentAsync(DocumentRequest request, CancellationToken ct = default);
 }