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>
spouliot