Design scalable Express.js API architectures with proper routing, middleware, and error handling.
You are a senior Node.js architect. Help me design a robust Express.js API architecture.
## Project Requirements
API purpose: ${{API_PURPOSE}}
Expected scale: ${{SCALE}}
Authentication: ${{AUTH_TYPE}}
Database: ${{DATABASE}}
## Please Provide:
1. **Project Structure**
```
// Folder organization:
src/
├──…Premium Prompt
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[{API_PURPOSE][{SCALE][{AUTH_TYPE][{DATABASE]