feat: MCP-Tools für Fach-/Kompetenzverwaltung + Kompetenzzuordnung zu Lessons (Nutzer-Nachtrag)

Neue CompetencyTools.cs: create/update/delete_subject, create/update/delete_competency_domain,
add/update/remove_competency_item, get_subjects, get_competency_catalog. delete_subject und
delete_competency_domain als weitere Destructive-Ausnahmen neben delete_lesson (kein Papierkorb).

Lesson bekommt ein neues Competencies-Feld (analog zu Unit.Competencies) sowie
add_lesson_competency/remove_lesson_competency in LessonPlanTools.cs.

ICompetencyDomainRepository um GetBySubject() erweitert, um den Katalog eines Fachs ohne
bekannte Klassenstufe abzufragen.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

LehrerApp.Tests/CompetencyCatalogImportServiceTests.cs

@@ -181,6 +181,8 @@ public sealed class CompetencyCatalogImportServiceTests
 
         public List<CompetencyDomain> GetBySubjectAndGrade(Guid subjectId, int gradeLevel) =>
             Current.Where(x => x.SubjectId == subjectId && x.GradeLevel == gradeLevel).ToList();
+        public List<CompetencyDomain> GetBySubject(Guid subjectId) =>
+            Current.Where(x => x.SubjectId == subjectId).ToList();
         public CompetencyDomain? GetById(Guid id) => Current.FirstOrDefault(x => x.Id == id);
 
         public void Save(CompetencyDomain domain) => Current.Add(domain);