A 58-year-old woman from Mumbai with a 3-year history of diabetic neuropathic pain (DNP) in both feet presents with inadequate relief on gabapentin 1800 mg/day divided into three doses. Pain severity is 7/10 despite compliance. She has mild renal impairment (eGFR 45 mL/min/1.73 m²) and is on metformin, lisinopril, and atorvastatin. She denies depression or significant anxiety. What is the most appropriate next step in managing her chronic neuropathic pain?