Change AgeGroup ID type from int to string and add migration

Refactored AgeGroupController, AgeGroupService, and IAgeGroupService to use string IDs instead of int. Added initial Entity Framework migration and database files to support AltersGruppe with string primary key.

API/Migrations/20251207192702_Init.cs

@@ -0,0 +1,35 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace API.Migrations
+{
+    /// <inheritdoc />
+    public partial class Init : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateTable(
+                name: "Altersgruppen",
+                columns: table => new
+                {
+                    Id = table.Column<string>(type: "TEXT", maxLength: 26, nullable: false),
+                    Name = table.Column<string>(type: "TEXT", maxLength: 100, nullable: false),
+                    StartingAge = table.Column<int>(type: "INTEGER", nullable: false),
+                    EndingAge = table.Column<int>(type: "INTEGER", nullable: false)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_Altersgruppen", x => x.Id);
+                });
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(
+                name: "Altersgruppen");
+        }
+    }
+}