assign_ticket.php: preserve string ticket ID (ctype_digit validation)
instead of (int) cast for consistent audit logging and URL generation.
delete_attachment.php: use string ticket_id from DB for the upload
directory path — (int) cast was stripping leading zeros, causing
the wrong path (/uploads/123456/) instead of /uploads/000123456/.
Also pass raw string to getTicketById() to let TicketModel handle
type coercion.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>