Design a comprehensive microservices architecture for a global fintech platform that must handle 50,000 transactions per second with a latency of under 100ms. Your design must address eventual consistency across three geographical regions, fault tolerance using circuit breakers, and a strategy for database sharding to prevent hotspots. Explain your choice of inter-service communication protocols (e.g., gRPC vs. REST) and how you handle idempotency in payment processing.