Add duplicate invoice detection and warning message

Added a warning message in `Upload.cshtml` to notify users when a duplicate invoice is detected. Introduced the `IsDuplicate` property in `UploadModel` to track duplicates and updated the `OnPostAsync` method to set this property based on the `ImportedAt` timestamp.

Enhanced the `ImportAsync` method in `ZugferdImportService` to include duplicate detection by checking the database for invoices with the same `InvoiceNumber` and `SellerTaxId`. If a duplicate is found, it logs a warning and returns the existing invoice.

Updated `ImportAsync` to accept an optional `guidelineId` parameter and added logging for duplicate detection and successful imports.

DXApp.TemplateKitProject/Services/ZugferdImportService.cs

@@ -1,5 +1,6 @@
 using DXApp.TemplateKitProject.Data;
 using DXApp.TemplateKitProject.Models;
+using Microsoft.EntityFrameworkCore;
 
 namespace DXApp.TemplateKitProject.Services;
 
@@ -9,7 +10,7 @@ public class ZugferdImportService(
     AppDbContext db,
     ILogger<ZugferdImportService> logger)
 {
-    public async Task<ZugferdInvoice?> ImportAsync(Stream pdfStream, string sourceType)
+    public async Task<ZugferdInvoice?> ImportAsync(Stream pdfStream, string sourceType, string guidelineId = "")
     {
         var xml = extractor.ExtractXml(pdfStream);
 
@@ -20,11 +21,31 @@ public class ZugferdImportService(
         }
 
         var invoice = parser.Parse(xml);
+
+        // Duplikatprüfung
+        var duplicate = await db.ZugferdInvoices.FirstOrDefaultAsync(i =>
+            i.InvoiceNumber == invoice.InvoiceNumber &&
+            i.SellerTaxId == invoice.SellerTaxId);
+
+        if (duplicate is not null)
+        {
+            logger.LogWarning(
+                "Duplikat erkannt: Rechnung '{Number}' von '{Seller}' existiert bereits (ID: {Id}).",
+                invoice.InvoiceNumber, invoice.SellerName, duplicate.Id);
+            return duplicate;
+        }
+
         invoice.SourceType = sourceType;
+        invoice.GuidelineId = guidelineId;
+        invoice.ImportedAt = DateTime.UtcNow;
 
         db.ZugferdInvoices.Add(invoice);
         await db.SaveChangesAsync();
 
+        logger.LogInformation(
+            "Rechnung '{Number}' von '{Seller}' importiert (ID: {Id}).",
+            invoice.InvoiceNumber, invoice.SellerName, invoice.Id);
+
         return invoice;
     }
 }