A 35-year-old woman from Mumbai presents with a 3-month history of chronic diarrhoea, perianal fistulae, and abdominal pain localised to the right lower quadrant. Colonoscopy shows patchy areas of inflammation with normal intervening mucosa ('skip lesions') extending from the terminal ileum into the caecum. Biopsy reveals transmural inflammation with non-caseating granulomas in the submucosa and muscularis propria. She has already failed mesalamine 2.4 g/day and a 6-week course of prednisolone (now steroid-dependent). What is the most appropriate next step in management?