Ensure demo-seeded invoice PDFs are present on storage

[thibaudgg]

Why

Production AppSignal exception incident #370 is an ActiveStorage::FileNotFoundError on tenant demo-de (blob id 131). Admin invoice show preview (_invoice_preview.html.erb) calls invoice.pdf_file.preview when invoice.pdf_current? && !invoice.processing?. The attachment row existed; the S3 object key did not.

This is separate from:

• PR Return 404 when Active Storage representation blobs are missing #173 (merged) — FileNotFoundError → 404 on representations. Defense in depth for the request path; it does not stop the seeder from creating orphans.
• pdf_stale / Send-button fixes (6cf1408, b8a4a3a, 1811e08) — those keep Other invoices sendable around Active Storage updated_at touches. pdf_current? is !pdf_stale? when attached, so an orphan blob with pdf_stale: false still offers the preview. Timing alone would not have prevented #370.

ChoresJob#purge_unattached_active_storage_blobs! only purges unattached blobs older than a week, so this attached orphan stays.

What

Demo seed wraps invoice.process! → attach_pdf in a transaction (create_other_invoices!, seed_historical_shop_orders!). On Rails 8.1, Active Storage persists blob/attachment rows in after_save and uploads in after_commit. A reload on the same invoice instance before commit (shop historical orders do this) clears attachment_changes and skips the upload — DB row, missing object key (Rails #57222).

After the seed transaction:

1. Check each attached invoice pdf_file with blob.service.exist?(blob.key) (works for tenant_object_store / TenantS3 in production and Disk in test).
2. Re-call Invoice#attach_pdf outside the transaction when the key is missing, so the upload is synchronous and pdf_stale stays false.

mark_deliveries_delivered! still rescues FileNotFoundError as a safety net (same class of race, added in 066e96c). ChoresJob purge rules are unchanged.

Tests

• Re-attach when the blob row exists but the file does not; pdf_current? / sendability stay true.
• Leave present PDFs in place (same blob id).

Do not merge or deploy from this PR. Leave the AppSignal incident open.