Work by another agent on the PO send flow. Adds a 'Resend' CTA
on the order detail page for POs that were already sent
(operator wants to send another copy to the supplier), plus
acceptance of the same flow from the modal. Includes:
- purchase-orders-service: accept 'sent' as a resend-from state
(in addition to 'draft' and 'mail_failed')
- send-po-modal: title flips to 'Resend Purchase Order ...' when
the PO was previously sent; 'Send' button becomes 'Resend'
- send-po-modal.test: covers the resend path
- orders/[id]/page: shows Resend button in the same sendable
state group as Send (was draft / mail_failed, now also 'sent')
- purchase-orders pdf route: updates docstring to reflect that
status is the outbound-email pipeline's responsibility
- stock-purchases pdf route: download URL uses file proxy for
admin-auth compatibility
Co-authored-by: Orca <help@stably.ai>
