A 42-year-old woman with cirrhosis (Child-Pugh C) presents with fever (38.8°C), abdominal pain, and altered mental status. On examination, she is hypotensive (BP 82/48 mmHg), tachycardic (HR 126/min), and has diffuse abdominal tenderness without guarding. Ascitic fluid analysis shows: WBC 350/μL (neutrophils 80%), protein 1.2 g/dL, glucose 45 mg/dL, and culture is pending. Serum creatinine is 2.8 mg/dL (baseline 1.2), and lactate is 5.1 mmol/L. Which of the following is the most appropriate next step in management?