A 28-year-old male with a 5-year history of duodenal bulb ulcers, recurrent melena, and intermittent proton pump inhibitor (PPI) therapy presents with hematemesis (600ml total) 4 hours ago, melena (200g total), marked dizziness, sweating, palpitations, and syncope upon sitting up. Hemoglobin (Hb) is 68g/L. The most urgent intervention is: