Add support for invoice attachments

Introduced the `InvoiceAttachment` entity and its relationship with `ZugferdInvoice` to manage extracted invoice attachments. Updated `AppDbContext` and added a migration to create the `InvoiceAttachments` table with cascading delete behavior and an index for optimized queries.

Enhanced the UI to display attachments in `Details.cshtml`, including file type icons, file size, and extraction date. Added a new `ViewAttachment` page to render or download attachments based on their type, with support for XML, plain text, images, and downloads.

Implemented `AttachmentViewerService` to determine viewer types and MIME types for attachments. Registered the service in the DI container. Updated `Upload.cshtml.cs` to save extracted attachments to the database.

Improved user experience with syntax highlighting for XML files and appropriate messages for unsupported file types.

DXApp.TemplateKitProject/Pages/Invoices/ViewAttachment.cshtml.cs

@@ -0,0 +1,42 @@
+using DXApp.TemplateKitProject.Services;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+using System.Text;
+
+namespace DXApp.TemplateKitProject.Pages.Invoices;
+
+public class ViewAttachmentModel(AttachmentViewerService viewerService) : PageModel
+{
+    public string FileName { get; private set; } = string.Empty;
+    public AttachmentViewerType ViewerType { get; private set; }
+    public string? TextContent { get; private set; }
+
+    public IActionResult OnGet(string filePath)
+    {
+        if (string.IsNullOrEmpty(filePath) || !System.IO.File.Exists(filePath))
+            return NotFound();
+
+        FileName = Path.GetFileName(filePath);
+        ViewerType = viewerService.DetermineViewerType(FileName);
+
+        // Für Text/XML: Inhalt laden
+        if (ViewerType == AttachmentViewerType.Xml || ViewerType == AttachmentViewerType.Text)
+        {
+            TextContent = System.IO.File.ReadAllText(filePath, Encoding.UTF8);
+        }
+
+        return Page();
+    }
+
+    public IActionResult OnGetDownload(string filePath)
+    {
+        if (string.IsNullOrEmpty(filePath) || !System.IO.File.Exists(filePath))
+            return NotFound();
+
+        var fileName = Path.GetFileName(filePath);
+        var mimeType = viewerService.GetMimeType(fileName);
+        var bytes = System.IO.File.ReadAllBytes(filePath);
+
+        return File(bytes, mimeType, fileName);
+    }
+}