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/tests/BigRegister.Tests/RuleTests.cs

@@ -31,6 +31,33 @@ public class DocumentRuleTests
         var c = DocumentRules.Find("registratie", "diploma");
         Assert.Null(DocumentRules.RejectUpload(c, "application/pdf", 5L * 1024 * 1024));
     }
+
+    private static IReadOnlyList<string> Ids(string? herkomst, string? taalvaardigheid) =>
+        DocumentRules.CategoriesFor("registratie", herkomst, taalvaardigheid).Select(c => c.CategoryId).ToList();
+
+    [Fact]
+    public void First_load_has_no_diploma_upload() => // no diploma chosen yet
+        Assert.Equal(new[] { "identiteit" }, Ids(null, null));
+
+    [Fact]
+    public void Manual_diploma_needs_a_diploma_upload() =>
+        Assert.Equal(new[] { "diploma", "identiteit" }, Ids("handmatig", null));
+
+    [Fact]
+    public void Duo_diploma_skips_diploma_upload() =>
+        Assert.DoesNotContain("diploma", Ids("duo", null));
+
+    [Fact]
+    public void Confirmed_dutch_proficiency_requires_taalvaardigheid_proof() =>
+        Assert.Contains("taalvaardigheid", Ids("handmatig", "ja"));
+
+    [Fact]
+    public void Unconfirmed_proficiency_requires_no_taalvaardigheid_proof() =>
+        Assert.DoesNotContain("taalvaardigheid", Ids("handmatig", "nee"));
+
+    [Fact]
+    public void Find_resolves_taalvaardigheid_for_upload_validation() =>
+        Assert.NotNull(DocumentRules.Find("registratie", "taalvaardigheid"));
 }
 
 public class HerregistratieRuleTests