Optimize invoice performance with indexes and DTO

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.

DXApp.TemplateKitProject/Pages/Invoices/Index.cshtml

@@ -12,7 +12,7 @@
 }
 else
 {
-    @(Html.DevExtreme().DataGrid<DXApp.TemplateKitProject.Models.ZugferdInvoice>()
+    @(Html.DevExtreme().DataGrid<DXApp.TemplateKitProject.Models.ZugferdInvoiceListDto>()
         .DataSource(Model.Invoices)
         .KeyExpr("Id")
         .ShowBorders(true)