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.).

首先检查现有 backend、AI 和 frontend 服务的入口点，然后在当前 setup 可用时运行它。添加 issue 中描述的 Docker Compose 配置、frontend nginx 配置、README 说明和 .env.example。完成标准是：docker compose up --build 能在指定端口启动全部三个服务，上传内容和数据能够持久化，并且 README 涵盖演示、故障排除、端口更改和持久化。

由索引模型根据 Issue 内容生成。