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.Application/DTOs/GiftCertificate/GiftCertificateDtos.cs

@@ -16,6 +16,7 @@ public class GiftCertificateListDto
     public GiftCertificateStatus Status { get; set; }
     public DateTime IssueDate { get; set; }
     public DateTime? ExpiryDate { get; set; }
+    public Guid? BatchId { get; set; }
 }
 
 public class GiftCertificateDto : GiftCertificateListDto