Switch AltersGruppe ID to ULID and add config

Changed AltersGruppe model ID from int to string and now generates ULID automatically. Added AltersGruppeConfiguration for EF Core mapping. Updated ApplicationDbContext to apply configurations from assembly. Removed obsolete migration files and database artifacts. Added Ulid package dependency.

API/Models/Internal/Altersgruppen/AltersGruppe.cs

@@ -4,9 +4,15 @@ namespace API.Models.Internal.Altersgruppen
 {
     public class AltersGruppe
     {
-        public int Id { get; set; }
+        public string Id { get; set; }
         public string Name { get; set; }
         public int StartingAge { get; set; }
         public int EndingAge { get; set; }
+
+        // Constructor für automatische ULID-Generierung
+        public AltersGruppe()
+        {
+            Id = Ulid.NewUlid().ToString();
+        }
     }
 }