Add duplicate invoice detection and warning message

Added a warning message in `Upload.cshtml` to notify users when a duplicate invoice is detected. Introduced the `IsDuplicate` property in `UploadModel` to track duplicates and updated the `OnPostAsync` method to set this property based on the `ImportedAt` timestamp.

Enhanced the `ImportAsync` method in `ZugferdImportService` to include duplicate detection by checking the database for invoices with the same `InvoiceNumber` and `SellerTaxId`. If a duplicate is found, it logs a warning and returns the existing invoice.

Updated `ImportAsync` to accept an optional `guidelineId` parameter and added logging for duplicate detection and successful imports.

DXApp.TemplateKitProject/Pages/Invoices/Upload.cshtml

@@ -104,5 +104,12 @@
             <tr><th>Importiert am</th><td>@Model.ImportedInvoice.ImportedAt.ToString("dd.MM.yyyy HH:mm")</td></tr>
         </table>
         <div class="alert alert-success mt-2">✔ Rechnung wurde in der Datenbank gespeichert (ID: @Model.ImportedInvoice.Id)</div>
+        @if (Model.IsDuplicate)
+        {
+            <div class="alert alert-warning mt-2">
+                ⚠️ <strong>Duplikat:</strong> Diese Rechnung wurde bereits importiert (ID: @Model.ImportedInvoice!.Id).
+                Es wurde kein neuer Eintrag angelegt.
+            </div>
+        }
     }
 }