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.
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.
