Add local development setup documentation

[SL-Mar]

Problem

The project has production Docker setup but no clear development guide. Users must know to set AUTH_ENABLED=false and RATE_LIMIT_ENABLED=false in .env for local dev, but this isn't documented.

Steps to fix

1. Add DEVELOPMENT.md covering:
   • Prerequisites (Docker, Node.js, Python versions)
   • Quick start for local development
   • Environment setup (dev vs production mode)
   • How to run tests
   • How to test auth and rate limiting locally
2. Consider adding docker-compose.dev.yml with relaxed settings

Files

• New: DEVELOPMENT.md
• .env.example (add dev mode comments)