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.Desktop/Services/Mcp/Tools/Dto.cs

@@ -30,7 +30,7 @@ public record LessonAttachmentDto(string StorageId, string FileName, long SizeBy
 
 public record LessonDto(
     Guid Id, Guid UnitId, Guid GroupId, DateOnly Date, int? LessonNumber, string Topic,
-    string? Homework, LessonStatus Status, List<LessonPhaseDto> Phases,
+    string? Homework, LessonStatus Status, List<string> Competencies, List<LessonPhaseDto> Phases,
     List<LessonAttachmentDto> Attachments);
 
 /// <summary>Ergebnis von "download_lesson_attachment": Inhalt Base64-kodiert, weil MCP-Tool-Antworten
@@ -45,6 +45,14 @@ public record UnitDto(
 
 public record LessonPlanResultDto(List<UnitDto> Units, List<LessonDto> Lessons);
 
+public record SubjectDto(Guid Id, string Name, string ShortName);
+
+public record CompetencyItemDto(Guid Id, string Code, string Description, int SortOrder);
+
+public record CompetencyDomainDto(
+    Guid Id, Guid SubjectId, int GradeLevel, string Name, string Code, int SortOrder,
+    List<CompetencyItemDto> Items);
+
 public record GroupMembershipDto(
     Guid Id, Guid StudentId, Guid GroupId, MembershipPeriod Period,
     DateOnly? JoinedAt, DateOnly? LeftAt, Niveau? Niveau);