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/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>