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

DXApp.TemplateKitProject/Pages/Invoices/Upload.cshtml

@@ -42,6 +42,15 @@
             }
         </div>
 
+        @* PDF/A-Konformitätsstufe anzeigen *@
+        <div class="alert @(Model.Result.PdfAWarning ? "alert-warning" : "alert-info") mt-2">
+            <strong>PDF/A-Konformität:</strong> @Model.Result.PdfALevel
+            @if (Model.Result.PdfAWarning)
+            {
+                <span> – ⚠️ ZUGFeRD-Rechnungen müssen PDF/A-3b sein.</span>
+            }
+        </div>
+
         <table class="table table-sm table-bordered">
             <thead class="table-light">
                 <tr>

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();