Senior Full-Stack Developer React Node.js, PostgreSQL, Backend Systems & API Integration

We need a senior full-stack developer to implement updates to an existing SMS-based platform ahead of a research study launch. The system is already live and running. Work involves modifying the backend logic and conversation scripts, adjusting the message flows for a structured cohort format, updating the data model to track participants and session history, and building out the internal views the team uses to review responses. Stack: Node.js backend, PostgreSQL, React and Next.js front end, OpenAI API, Twilio API Integration for SMS delivery. Looking for someone comfortable taking over an existing codebase, working from written change requests, and shipping quickly without heavy oversight.