feat(fp): WP-23 — org-template backend + admin role

Second template axis (org identity: letterhead, footer, signature,
margins) server-side: OrgTemplateStore with JSON version history,
publish/rollback, sent-brief version pinning, admin role + capability,
5 admin endpoints, org-logo upload category. FE seam widened only
(Role/Capability unions, interceptor); WP-24/26 consume it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/src/BigRegister.Api/Domain/Documents/DocumentCategory.cs

@@ -19,6 +19,7 @@ public static class DocumentRules
 {
   private static readonly string[] Pdf = { "application/pdf" };
   private static readonly string[] PdfImage = { "application/pdf", "image/jpeg", "image/png" };
+  private static readonly string[] Image = { "image/jpeg", "image/png" };
 
   private static readonly DocumentCategory Diploma = new("diploma", "Diplomabewijs",
       "Upload uw diploma als PDF-bestand.", true, Pdf, 10, false, false);
@@ -42,6 +43,13 @@ public static class DocumentRules
             new DocumentCategory("nascholing", "Nascholingscertificaten",
                 "Upload uw nascholingscertificaten (optioneel).", false, PdfImage, 10, true, true),
         },
+    // WP-23: the admin's org-template logo rides the same upload machinery as the
+    // wizard documents — one category under its own "wizard" id.
+    "org-template" => new[]
+    {
+            new DocumentCategory("org-logo", "Organisatielogo",
+                "Upload het logo van de organisatie (PNG of JPG).", false, Image, 1, false, false),
+        },
     _ => Array.Empty<DocumentCategory>(),
   };