Full-Stack Developer for Cleaning SaaS w AI

We have an existing web app with a React/Next.js and TypeScript frontend, Python/FastAPI backend, and PostgreSQL. We need a senior full-stack developer to fix and improve the current codebase, complete unfinished user flows, add reliable API integrations and an AI agent feature, and launch the next release. The work includes backend API development, frontend fixes, authentication and permissions, LLM/RAG integration with product data, error handling, testing, and deployment. We want someone who has shipped production SaaS applications and can own features end to end. Please share similar React/Python, AI agent, or API integration work and how you would approach the first sprint.