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.Api/Program.cs

@@ -40,6 +40,15 @@ app.MapPost("/zaken/reference", async (ZaakReferenceRequest body, AclService acl
     return Results.Ok(new { reference });
 });
 
+// Store an uploaded diploma against a zaak (S-10b): the domain sends the file as base64; the ACL
+// creates the ZGW enkelvoudiginformatieobject and relates it to the zaak (§8.1). Returns its URL.
+app.MapPost("/documenten", async (StoreDocumentRequest body, AclService acl, CancellationToken ct) =>
+{
+    var url = await acl.StoreDiplomaAsync(
+        new Uri(body.ZaakUrl), Convert.FromBase64String(body.ContentBase64), body.FileName, body.ContentType, ct);
+    return Results.Ok(new { informatieobjectUrl = url.ToString() });
+});
+
 app.Run();
 
 public sealed record OpenZaakRequest(string Bsn, string Reference);
@@ -48,4 +57,6 @@ public sealed record SetStatusRequest(string ZaakUrl);
 
 public sealed record ZaakReferenceRequest(string ZaakUrl);
 
+public sealed record StoreDocumentRequest(string ZaakUrl, string ContentBase64, string FileName, string ContentType);
+
 public partial class Program;