Add BatchId to GiftCertificate for persistent bulk batch tracking

BatchId (Guid?) is stamped on every certificate in a bulk run so the batch
is permanently addressable. BulkResult is now a bookmarkable GET by batchId
rather than TempData, so users can return to re-download at any time.
BatchDownloadPdf is a GET link (no form POST needed). Index shows a Batch
badge on bulk certs that links directly back to the batch result page.

Migration: AddGiftCertificateBatchId

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Core/Entities/GiftCertificate.cs

@@ -32,6 +32,9 @@ public class GiftCertificate : BaseEntity
     /// <summary>Set when this GC was sold via an invoice line item.</summary>
     public int? SourceInvoiceItemId { get; set; }
 
+    /// <summary>Groups all certificates created in a single bulk run. Null for individually issued certs.</summary>
+    public Guid? BatchId { get; set; }
+
     // Navigation
     public virtual Customer? RecipientCustomer { get; set; }
     public virtual Customer? PurchasingCustomer { get; set; }