feat(stamdata): extract policy-question text into Stamdata

Move the geldigheidsvragen wording out of DiplomaRules into
Stamdata.PolicyQuestions (business-editable text, config-as-code); DiplomaRules
keeps only the rule of which questions apply. Extend StamdataValidationTests
(no blank id/wording, distinct ids in the manual set) and update ADR-0004.

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

backend/tests/BigRegister.Tests/StamdataValidationTests.cs

@@ -38,4 +38,21 @@ public class StamdataValidationTests
     Assert.NotEmpty(professions);
     Assert.Equal(professions.Count, professions.Distinct().Count());
   }
+
+  [Fact]
+  public void Every_policy_question_has_an_id_and_wording()
+  {
+    Assert.All(PolicyQuestions.All, q =>
+    {
+      Assert.False(string.IsNullOrWhiteSpace(q.Id), "A policy question has a blank id.");
+      Assert.False(string.IsNullOrWhiteSpace(q.Vraag), $"Policy question '{q.Id}' has blank wording.");
+    });
+  }
+
+  [Fact]
+  public void Manual_question_set_has_distinct_ids() // no field is asked twice
+  {
+    var ids = PolicyQuestions.ManualSet.Select(q => q.Id).ToList();
+    Assert.Equal(ids.Count, ids.Distinct().Count());
+  }
 }