feat: add student master data import

LehrerApp.Data/Repositories/AllRepositories.cs

@@ -37,6 +37,8 @@ public class StudentRepository(LiteDbContext db) : IStudentRepository
         db.Documentation.Count(d => d.StudentId == studentId));
     public void Save(Student s)
     {
+        s.ExternalIds ??= [];
+        s.Contacts ??= [];
         s.UpdatedAt = DateTime.UtcNow;
         db.Students.Upsert(s);
         db.OnChange?.Invoke(nameof(Student), s.Id.ToString(), "Save", s);