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/Models/InvoiceAttachment.cs

@@ -0,0 +1,18 @@
+namespace DXApp.TemplateKitProject.Models;
+
+/// <summary>
+/// Extrahierte Anhänge einer ZUGFeRD-Rechnung (gespeichert in DB)
+/// </summary>
+public class InvoiceAttachment
+{
+    public int Id { get; set; }
+    public int ZugferdInvoiceId { get; set; }
+    public string OriginalFileName { get; set; } = string.Empty;
+    public string SavedFilePath { get; set; } = string.Empty;
+    public long FileSizeBytes { get; set; }
+    public bool IsZugferdXml { get; set; }
+    public DateTime ExtractedAt { get; set; }
+
+    // Navigation Property
+    public ZugferdInvoice? ZugferdInvoice { get; set; }
+}