A 52-year-old man with a 15-year history of alcohol use disorder presents to the emergency department with haematemesis and melaena. On examination, he is hypotensive (BP 88/54 mmHg), tachycardic (HR 118/min), and has clinical signs of chronic liver disease including spider angiomas, palmar erythema, and ascites. Oesophagogastroduodenoscopy (OGD) confirms bleeding oesophageal varices. After initial resuscitation and variceal ligation, the patient stabilises. However, 48 hours later, he develops acute kidney injury (serum creatinine rises from 1.2 to 3.8 mg/dL), oliguria, and worsening encephalopathy despite adequate haemostasis. Urine sodium is <10 mEq/L and urine osmolality is 600 mOsm/kg. What is the most likely diagnosis?