A 32-year-old woman with known primary adrenal insufficiency (Addison disease) taking hydrocortisone 20 mg/day and fludrocortisone 100 µg/day presents with vomiting and diarrhoea and is unable to retain oral medications. She is hypotensive at 85/50 mmHg, tachycardic and appears shocked. What is the most appropriate immediate management in the emergency department?