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/Program.cs

@@ -18,6 +18,7 @@ builder.Services.AddScoped<PdfResultPackageService>();
 builder.Services.AddScoped<ZugferdExtractorService>();
 builder.Services.AddScoped<ZugferdParserService>();
 builder.Services.AddScoped<ZugferdImportService>();
+builder.Services.AddScoped<AttachmentViewerService>();
 
 var app = builder.Build();