Enhance user modeling and recommendation services with async support and logging

Refactor environment variable handling and user modeling prompt

Refactor model loading and startup prewarm logic

Enhance Dockerfile and application startup process with prewarm functionality

Add agent workflow documentation and refactor user modeling and recommendation services

Update README.md with detailed configuration, deliverables checklist, API endpoints, request bodies, local run instructions, Docker setup, and project layout for the DSN × BCT LLM Agent Challenge.