fix: web/app/api/chat/route.ts

fix: upload actual chat/route.ts content — FastAPI fallback, session docs, input validation

🛡️ v4.1: Fix all critical bugs, security issues, and performance problems (#2)

feat: add user authentication checks and improve navigation with logout functionality

v4.0: Fix /api/chat — call Gradio Space chatbot endpoint

v4.0: Add web /api/chat route for RAG chatbot