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/src/BigRegister.Api/Domain/Diplomas/DiplomaRules.cs

@@ -19,44 +19,20 @@ public static class DiplomaRules
   /// <summary>Professions a user may declare for a manual (unlisted) diploma.</summary>
   public static IReadOnlyList<string> ManualProfessions() => Professions.All();
 
-  // --- Policy questions (geldigheidsvragen) ---
-
-  private static readonly PolicyQuestion NlTaalEngelstalig = new(
-      "nl-taalvaardigheid",
-      "Uw opleiding was Engelstalig. Beheerst u de Nederlandse taal op het vereiste niveau (B2)?",
-      QuestionType.JaNee);
-
-  private static readonly PolicyQuestion NlTaalManual = new(
-      "nl-taalvaardigheid",
-      "Beheerst u de Nederlandse taal op het vereiste niveau (B2)?",
-      QuestionType.JaNee);
-
-  private static readonly PolicyQuestion DiplomaErkend = new(
-      "diploma-erkend",
-      "Is uw diploma erkend door de Nederlandse overheid (bijv. via Nuffic)?",
-      QuestionType.JaNee);
-
-  private static readonly PolicyQuestion Toelichting = new(
-      "toelichting",
-      "Geef een korte toelichting op uw diploma en opleiding.",
-      QuestionType.Tekst);
-
   /// <summary>
-  /// RULE: an English-language diploma requires proof of Dutch proficiency (B2).
-  /// Add a question here to apply it to a (set of) diploma(s) — a single backend
-  /// change, no frontend change.
+  /// RULE: an English-language diploma requires proof of Dutch proficiency (B2). Which
+  /// question applies is the rule; its wording is stamdata in <see cref="PolicyQuestions"/>.
   /// </summary>
   public static IReadOnlyList<PolicyQuestion> QuestionsFor(Diploma d)
   {
     var questions = new List<PolicyQuestion>();
     if (d.Engelstalig)
-      questions.Add(NlTaalEngelstalig);
+      questions.Add(PolicyQuestions.NlTaalEngelstalig);
     return questions;
   }
 
   /// <summary>
   /// RULE: a manual diploma is unverified, so the strictest (maximal) set applies.
   /// </summary>
-  public static IReadOnlyList<PolicyQuestion> ManualQuestions() =>
-      new[] { NlTaalManual, DiplomaErkend, Toelichting };
+  public static IReadOnlyList<PolicyQuestion> ManualQuestions() => PolicyQuestions.ManualSet;
 }