Optimize invoice performance with indexes and DTO

Added indexes for `ImportedAt` and a composite key on
`InvoiceNumber` and `SellerTaxId` to improve query performance.
Updated `AppDbContext` and EF Core migrations to reflect these
changes. Introduced `ZugferdInvoiceListDto` to optimize memory
usage by excluding large fields like `RawXml`. Updated the
frontend (`Index.cshtml`) and backend (`Index.cshtml.cs`) to use
the new DTO for better performance.

DXApp.TemplateKitProject/Migrations/20260602114123_AddPerformanceIndexes.cs

@@ -0,0 +1,72 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace DXApp.TemplateKitProject.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddPerformanceIndexes : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AlterColumn<string>(
+                name: "SellerTaxId",
+                table: "ZugferdInvoices",
+                type: "nvarchar(450)",
+                nullable: true,
+                oldClrType: typeof(string),
+                oldType: "nvarchar(max)",
+                oldNullable: true);
+
+            migrationBuilder.AlterColumn<string>(
+                name: "InvoiceNumber",
+                table: "ZugferdInvoices",
+                type: "nvarchar(450)",
+                nullable: true,
+                oldClrType: typeof(string),
+                oldType: "nvarchar(max)",
+                oldNullable: true);
+
+            migrationBuilder.CreateIndex(
+                name: "IX_ZugferdInvoices_ImportedAt",
+                table: "ZugferdInvoices",
+                column: "ImportedAt");
+
+            migrationBuilder.CreateIndex(
+                name: "IX_ZugferdInvoices_InvoiceNumber_SellerTaxId",
+                table: "ZugferdInvoices",
+                columns: new[] { "InvoiceNumber", "SellerTaxId" });
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropIndex(
+                name: "IX_ZugferdInvoices_ImportedAt",
+                table: "ZugferdInvoices");
+
+            migrationBuilder.DropIndex(
+                name: "IX_ZugferdInvoices_InvoiceNumber_SellerTaxId",
+                table: "ZugferdInvoices");
+
+            migrationBuilder.AlterColumn<string>(
+                name: "SellerTaxId",
+                table: "ZugferdInvoices",
+                type: "nvarchar(max)",
+                nullable: true,
+                oldClrType: typeof(string),
+                oldType: "nvarchar(450)",
+                oldNullable: true);
+
+            migrationBuilder.AlterColumn<string>(
+                name: "InvoiceNumber",
+                table: "ZugferdInvoices",
+                type: "nvarchar(max)",
+                nullable: true,
+                oldClrType: typeof(string),
+                oldType: "nvarchar(450)",
+                oldNullable: true);
+        }
+    }
+}