Add environment variable configuration support

- Add .env.example file with API key placeholders
- Update README.md with .env file setup instructions
- Add dotenv loading in main.py for environment variables

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

.env.example

@@ -0,0 +1,2 @@
+ALPHA_VANTAGE_API_KEY=alpha_vantage_api_key_placeholder
+OPENAI_API_KEY=openai_api_key_placeholder