Add support for generating result PDFs

Introduced the `ResultFilePath` property in the `ZugferdInvoice` model to store the path of generated result PDFs. Added a new service, `PdfResultPackageService`, to create result PDFs by converting the original PDF to PDF/A-3b format and attaching a report file. Updated `Upload.cshtml` and `Upload.cshtml.cs` to handle and display the `ResultFilePath`.

Created a migration to add the `ResultFilePath` column to the database. Updated `Program.cs` to register the new service and added configuration sections in `appsettings.json` for input and output directories. Enhanced error handling and logging for better traceability.

DXApp.TemplateKitProject/Migrations/20260527094043_AddResultFilePath.cs

@@ -0,0 +1,28 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace DXApp.TemplateKitProject.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddResultFilePath : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<string>(
+                name: "ResultFilePath",
+                table: "ZugferdInvoices",
+                type: "nvarchar(max)",
+                nullable: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "ResultFilePath",
+                table: "ZugferdInvoices");
+        }
+    }
+}