Registratie: answer-driven required document uploads

Categories stay server-owned (ADR-0001); the FE sends its answers to
/uploads/categories and re-fetches reactively when they change:
- Diplomabewijs required only for a handmatig diploma (DUO is verified digitally;
  nothing required before a diploma is chosen).
- Bewijs Nederlandse taalvaardigheid required only when the applicant answers "ja"
  to the nl-taalvaardigheid (B2) policy question.
CategoriesFor(wizardId, diplomaHerkomst, taalvaardigheid) decides; Find uses the
maximal set so uploads still validate. CategoriesLoaded drops orphaned uploads
when a category disappears. Also: show the foreground-only upload banner only when
there is at least one category.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/BigRegister.Api/Program.cs

@@ -88,8 +88,8 @@ api.MapPost("/change-requests", (ChangeRequestRequest req, HttpContext ctx) =>
 // --- Document upload ---
 
 // Server-owned category config per wizard. The FE renders these; it never hardcodes.
-api.MapGet("/uploads/categories", (string wizardId) =>
-    new UploadCategoriesDto(DocumentRules.CategoriesFor(wizardId).Select(c => c.ToDto()).ToList()));
+api.MapGet("/uploads/categories", (string wizardId, string? diplomaHerkomst, string? taalvaardigheid) =>
+    new UploadCategoriesDto(DocumentRules.CategoriesFor(wizardId, diplomaHerkomst, taalvaardigheid).Select(c => c.ToDto()).ToList()));
 
 // Multipart upload. Hand-written on the FE (XHR for progress), so it is excluded
 // from the OpenAPI doc to keep the NSwag-generated client JSON-only. Validates type