A 42-year-old woman from North Carolina, USA, presents with a 5-day history of fever (39.8°C), severe headache, and myalgia. On examination, she has a petechial rash that began on her wrists and ankles 2 days ago and has now spread to involve the trunk, palms, and soles. She is hypotensive (BP 100/65) and tachycardic (HR 118/min). Laboratory findings show WBC 12,000/μL, platelets 45,000/μL, and elevated transaminases (AST 156 U/L, ALT 132 U/L). A skin biopsy shows vasculitis with rickettsiae within endothelial cells. What is the most likely diagnosis?