feat(openzaak): per-document-type confidentialiteit config (WP-59)

Drives the DRC upload's vertrouwelijkheidaanduiding from a new stamdata
table instead of the hardcoded "openbaar", following the existing
config-as-code pattern (ADR-0004). Adds the referential-integrity check
StamdataValidationTests was missing for the new table.

backend/src/BigRegister.Api/Zgw/OpenZaakDocumentSource.cs

@@ -3,6 +3,7 @@ using System.Text.Json.Serialization;
 using BigRegister.Api.Contracts;
 using BigRegister.Api.Data;
 using BigRegister.Domain.Authorization;
+using BigRegister.Stamdata;
 
 namespace BigRegister.Api.Zgw;
 
@@ -23,6 +24,15 @@ public sealed class OpenZaakDocumentSource(HttpClient http, ZgwTokenProvider tok
 {
   private readonly ZgwHttpClient zgw = new(http, tokens);
 
+  // WP-59: per-document-type confidentiality (stamdata, ADR-0004) — "openbaar" if the
+  // category isn't in the table, so an unconfigured category never fails the upload.
+  private static readonly IReadOnlyDictionary<string, string> ConfidentialiteitByCategory =
+      StamdataFile.Load<DocumentConfidentialiteit>("documentconfidentialiteit")
+          .ToDictionary(r => r.CategoryId, r => r.Vertrouwelijkheidaanduiding);
+
+  private static string ConfidentialiteitFor(string categoryId) =>
+      ConfidentialiteitByCategory.GetValueOrDefault(categoryId, "openbaar");
+
   // ponytail: sync-over-async — IDocumentSource is sync to match the local store + the
   // existing sync upload/submit endpoints, same reasoning as OpenZaakZaakSource.
   public UploadResponse Upload(
@@ -52,10 +62,7 @@ public sealed class OpenZaakDocumentSource(HttpClient http, ZgwTokenProvider tok
         Inhoud: Convert.ToBase64String(content),
         Informatieobjecttype: informatieobjecttypeUrl,
         Identificatie: doc.DocumentId,
-        // ponytail: hardcoded "openbaar" (public) — real usage would likely vary the
-        // confidentiality level per category (e.g. an identity document is more sensitive
-        // than a diploma); a fixed value is enough to prove the seam end-to-end.
-        Vertrouwelijkheidaanduiding: "openbaar"), caller);
+        Vertrouwelijkheidaanduiding: ConfidentialiteitFor(categoryId)), caller);
 
     DocumentStore.SetDrcUrl(doc.DocumentId, eio.Url);
     return new UploadResponse(doc.DocumentId, doc.LocalId);