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/Migrations/AppDbContextModelSnapshot.cs

@@ -22,6 +22,40 @@ namespace DXApp.TemplateKitProject.Migrations
 
             SqlServerModelBuilderExtensions.UseIdentityColumns(modelBuilder);
 
+            modelBuilder.Entity("DXApp.TemplateKitProject.Models.InvoiceAttachment", b =>
+                {
+                    b.Property<int>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("int");
+
+                    SqlServerPropertyBuilderExtensions.UseIdentityColumn(b.Property<int>("Id"));
+
+                    b.Property<DateTime>("ExtractedAt")
+                        .HasColumnType("datetime2");
+
+                    b.Property<long>("FileSizeBytes")
+                        .HasColumnType("bigint");
+
+                    b.Property<bool>("IsZugferdXml")
+                        .HasColumnType("bit");
+
+                    b.Property<string>("OriginalFileName")
+                        .HasColumnType("nvarchar(max)");
+
+                    b.Property<string>("SavedFilePath")
+                        .HasColumnType("nvarchar(max)");
+
+                    b.Property<int>("ZugferdInvoiceId")
+                        .HasColumnType("int");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("ZugferdInvoiceId")
+                        .HasDatabaseName("IX_InvoiceAttachments_ZugferdInvoiceId");
+
+                    b.ToTable("InvoiceAttachments");
+                });
+
             modelBuilder.Entity("DXApp.TemplateKitProject.Models.ZugferdInvoice", b =>
                 {
                     b.Property<int>("Id")
@@ -82,6 +116,22 @@ namespace DXApp.TemplateKitProject.Migrations
 
                     b.ToTable("ZugferdInvoices");
                 });
+
+            modelBuilder.Entity("DXApp.TemplateKitProject.Models.InvoiceAttachment", b =>
+                {
+                    b.HasOne("DXApp.TemplateKitProject.Models.ZugferdInvoice", "ZugferdInvoice")
+                        .WithMany("Attachments")
+                        .HasForeignKey("ZugferdInvoiceId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.Navigation("ZugferdInvoice");
+                });
+
+            modelBuilder.Entity("DXApp.TemplateKitProject.Models.ZugferdInvoice", b =>
+                {
+                    b.Navigation("Attachments");
+                });
 #pragma warning restore 612, 618
         }
     }