A 62-year-old man on ICU has severe hospital-acquired pneumonia. Sputum culture grows Klebsiella pneumoniae producing a carbapenemase (KPC). The organism is resistant to all carbapenems, cephalosporins, and piperacillin/tazobactam. It remains sensitive to ceftazidime/avibactam, colistin, and tigecycline. What is the recommended first-line antibiotic?