Act as a Security Architect. Perform a comprehensive threat model assessment for a new RESTful API for a peer-to-peer payment platform. Assume the API uses OAuth 2.0 and handles sensitive financial data. Identify potential threats across the STRIDE model (Spoofing, Tampering, Repudiation, Information Disclosure, Denial of Service, Elevation of Privilege). For each identified threat, propose a specific mitigation strategy involving cryptography, rate limiting, input validation, or infrastructure design. Prioritize the risks by severity.