test: split multi-assertion specs into single-behavior tests

One behavior per test across FE machine/store specs and backend endpoint
tests, so a failure names exactly what broke.

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

backend/tests/BigRegister.Tests/PreviewEndpointTests.cs

@@ -81,12 +81,17 @@ public class PreviewEndpointTests(TestWebApplicationFactory factory) : IClassFix
   }
 
   [Fact]
-  public async Task Proefbrief_is_admin_only_and_renders_the_draft_template()
+  public async Task Proefbrief_is_admin_only()
   {
     ResetStores();
     Assert.Equal(HttpStatusCode.Forbidden,
         (await _client.GetAsync($"/api/v1/admin/org-template/{Registers}/preview")).StatusCode);
+  }
 
+  [Fact]
+  public async Task Proefbrief_renders_the_draft_template_with_a_watermark()
+  {
+    ResetStores();
     var res = await _client.SendAsync(Req(HttpMethod.Get, $"/api/v1/admin/org-template/{Registers}/preview", role: "admin"));
     res.EnsureSuccessStatusCode();
     var html = await res.Content.ReadAsStringAsync();