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.
