Analyze a hypothetical monolithic e-commerce application with tightly coupled modules for Inventory, Orders, and Users. Design a migration strategy to transition this into microservices. Specifically address: 1) How to define bounded contexts for each service. 2) Strategies for data decomposition, including how to handle shared databases. 3) How to maintain data consistency across services using the Saga pattern. 4) The implementation of an API Gateway. Provide a step-by-step roadmap with potential risks and mitigation strategies.