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/Services/PdfAttachmentExtractorService.cs

@@ -26,6 +26,22 @@ public class PdfAttachmentExtractorService(
             using var processor = new PdfDocumentProcessor();
             processor.LoadDocument(pdfStream);
 
+            // PDF/A-Konformität prüfen
+            var compatibility = processor.Document.PdfACompatibility;
+            result.PdfALevel = compatibility switch
+            {
+                PdfACompatibility.None => "Kein PDF/A",
+                PdfACompatibility.PdfA1b => "PDF/A-1b",
+                PdfACompatibility.PdfA2b => "PDF/A-2b",
+                PdfACompatibility.PdfA3b => "PDF/A-3b",
+                _ => compatibility.ToString()
+            };
+            result.PdfAWarning = compatibility == PdfACompatibility.None;
+
+            logger.LogInformation(
+                "PDF '{FileName}': Konformität = {Level}",
+                sourceFileName, result.PdfALevel);
+
             // Fix: .ToList() → IEnumerable → List<T> mit Count-Property
             var attachments = processor.Document.FileAttachments.ToList();