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/swagger.json

@@ -296,6 +296,20 @@
             "schema": {
               "type": "string"
             }
+          },
+          {
+            "name": "diplomaHerkomst",
+            "in": "query",
+            "schema": {
+              "type": "string"
+            }
+          },
+          {
+            "name": "taalvaardigheid",
+            "in": "query",
+            "schema": {
+              "type": "string"
+            }
           }
         ],
         "responses": {