Docker Compose & Local Dev Setup for HomeOrganizer

Provide a reliable, documented Docker Compose setup to run all services for local development:
- Build containers for backend (Express), AI microservice (FastAPI), and frontend (React/nginx).
- Volume mounts for uploads and source code, ports exposed as: backend (4000), ai (8002), frontend (3000).
- Production-ready nginx config for frontend (proxies /api to backend in container network).
- Documentation for quickstart, troubleshooting, port changes, and data persistence.
- .env.example to make local overrides easy.

Acceptance:
- A single `docker compose up --build` brings up all services and persists uploads/data.
- README includes demo and troubleshooting instructions.
- Adaptable for future service addition (queue, S3, Postgres, etc.).