Design the backend for a marketplace that matches patients with therapists. The platform should support: - Patients searching for therapists by specialty, insurance, location, language, modality, price, and availability. - Therapist profile pages with credentials, specialties, ratings, session types, and accepted insurance. - Real-time or near-real-time appointment slot availability. - Booking, cancellation, waitlists, and reminders. - Ranking the most relevant therapists for a patient. - Preventing double booking and handling stale availability. - Region and licensure constraints for therapists. Discuss the API design, core services, data model, search and ranking strategy, booking consistency model, privacy and compliance considerations for sensitive health data, scaling approach, and key trade-offs.