Add PDF/A compliance checks and warnings

Added `PdfALevel` and `PdfAWarning` properties to the `PdfExtractionResult` class to store the PDF/A compliance level and indicate if a warning should be displayed for non-compliance.

Updated `Upload.cshtml` to display the PDF/A compliance level and conditionally show a warning message if the document is not PDF/A compliant.

Enhanced `PdfAttachmentExtractorService` to determine the PDF/A compliance level using `PdfDocumentProcessor`, map it to a string representation, and log the compliance level. Added logic to set `PdfAWarning` for non-compliant documents.

DXApp.TemplateKitProject/Models/PdfExtractionResult.cs

@@ -9,6 +9,14 @@ public class PdfExtractionResult
         Attachments.FirstOrDefault(a => a.IsZugferdXml);
 
     public bool HasZugferdXml => ZugferdXmlAttachment is not null;
+
+    // Welche PDF/A-Stufe hat das Dokument?
+    // Beispiel: "PDF/A-3b", "PDF/A-2b", oder "Kein PDF/A"
+    public string PdfALevel { get; set; } = string.Empty;
+
+    // Soll eine Warnung angezeigt werden?
+    // true wenn kein PDF/A → ZUGFeRD-Rechnungen müssen PDF/A-3b sein
+    public bool PdfAWarning { get; set; }
 }
 
 public class ExtractedAttachment