A 58-year-old man with a 10-year history of type 2 diabetes presents to the pain clinic with chronic neuropathic pain in both feet. He describes a burning sensation, numbness, and tingling that began 3 years ago and has progressively worsened despite tight glycemic control. He has tried paracetamol and NSAIDs without relief. On examination, he has diminished vibration sense and absent ankle reflexes bilaterally. His renal function is normal (eGFR 78 mL/min/1.73m²). Which of the following is the most appropriate first-line pharmacological agent for his neuropathic pain?