Add ZUGFeRD Guideline-ID extraction and display

Enhanced `PdfExtractionResult` with a new `ZugferdGuidelineId` property to store the ZUGFeRD Guideline-ID extracted from XMP metadata. Updated `Upload.cshtml` to display this information in the UI if available.

Implemented ZUGFeRD Guideline-ID extraction in `PdfAttachmentExtractorService` using a new helper method `ExtractGuidelineId`, which parses XMP metadata for known prefixes. Added logging for extracted Guideline-IDs and improved error handling with a `try-catch` block for metadata processing.

Fixed `FileAttachments` handling in `PdfAttachmentExtractorService` by converting to `List<T>` for proper `Count` property usage.

DXApp.TemplateKitProject/Services/PdfAttachmentExtractorService.cs

@@ -42,6 +42,24 @@ public class PdfAttachmentExtractorService(
                 "PDF '{FileName}': Konformität = {Level}",
                 sourceFileName, result.PdfALevel);
 
+            // ZUGFeRD Guideline-ID aus XMP-Metadaten lesen
+            try
+            {
+                var xmpData = processor.Document.Metadata?.Data;
+                if (!string.IsNullOrWhiteSpace(xmpData))
+                {
+                    result.ZugferdGuidelineId = ExtractGuidelineId(xmpData);
+                    if (!string.IsNullOrEmpty(result.ZugferdGuidelineId))
+                        logger.LogInformation(
+                            "PDF '{FileName}': Guideline-ID = {GuidelineId}",
+                            sourceFileName, result.ZugferdGuidelineId);
+                }
+            }
+            catch (Exception ex)
+            {
+                logger.LogWarning(ex, "XMP-Metadaten konnten nicht gelesen werden.");
+            }
+
             // Fix: .ToList() → IEnumerable → List<T> mit Count-Property
             var attachments = processor.Document.FileAttachments.ToList();
 
@@ -155,4 +173,34 @@ public class PdfAttachmentExtractorService(
                 ? $"Ja → {result.ZugferdXmlAttachment!.OriginalFileName}"
                 : "Nein");
     }
+
+    private static string ExtractGuidelineId(string xmpData)
+    {
+        // Bekannte ZUGFeRD/Factur-X Guideline-Präfixe
+        string[] knownPrefixes =
+        [
+            "urn:ferd:",
+        "urn:cen.eu:",
+        "urn:factur-x.",
+        "urn:zugferd:",
+        "urn:xoev-de:"
+        ];
+
+        // XMP ist XML → wir suchen nach dem Wert zwischen Tags
+        // Typisch: <ram:ID>urn:ferd:invoice:rc:comfort</ram:ID>
+        // oder:    <fx:ConformanceLevel>EN 16931</fx:ConformanceLevel>
+        foreach (var prefix in knownPrefixes)
+        {
+            var idx = xmpData.IndexOf(prefix, StringComparison.OrdinalIgnoreCase);
+            if (idx < 0) continue;
+
+            // Ende des Wertes finden (nächstes < Zeichen)
+            var end = xmpData.IndexOf('<', idx);
+            if (end < 0) end = xmpData.Length;
+
+            return xmpData[idx..end].Trim();
+        }
+
+        return string.Empty;
+    }
 }