Add support for invoice attachments

Introduced the `InvoiceAttachment` entity and its relationship with `ZugferdInvoice` to manage extracted invoice attachments. Updated `AppDbContext` and added a migration to create the `InvoiceAttachments` table with cascading delete behavior and an index for optimized queries.

Enhanced the UI to display attachments in `Details.cshtml`, including file type icons, file size, and extraction date. Added a new `ViewAttachment` page to render or download attachments based on their type, with support for XML, plain text, images, and downloads.

Implemented `AttachmentViewerService` to determine viewer types and MIME types for attachments. Registered the service in the DI container. Updated `Upload.cshtml.cs` to save extracted attachments to the database.

Improved user experience with syntax highlighting for XML files and appropriate messages for unsupported file types.

DXApp.TemplateKitProject/Pages/Invoices/Upload.cshtml.cs

@@ -75,6 +75,29 @@ public class UploadModel(
                         ImportedInvoice.ResultFilePath = ResultFilePath;
                         await db.SaveChangesAsync();
                     }
+
+                    // 4. Attachments in DB speichern
+                    if (Result.HasAttachments)
+                    {
+                        foreach (var attachment in Result.Attachments)
+                        {
+                            var invoiceAttachment = new InvoiceAttachment
+                            {
+                                ZugferdInvoiceId = ImportedInvoice.Id,
+                                OriginalFileName = attachment.OriginalFileName,
+                                SavedFilePath = attachment.SavedFilePath,
+                                FileSizeBytes = attachment.FileSizeBytes,
+                                IsZugferdXml = attachment.IsZugferdXml,
+                                ExtractedAt = DateTime.UtcNow
+                            };
+                            db.InvoiceAttachments.Add(invoiceAttachment);
+                        }
+                        await db.SaveChangesAsync();
+
+                        logger.LogInformation(
+                            "Rechnung '{InvoiceNumber}': {Count} Anhang/Anhänge in DB gespeichert.",
+                            ImportedInvoice.InvoiceNumber, Result.Attachments.Count);
+                    }
                 }
             }
         }