Structured API Request Logging

## 📋 Description
Implement comprehensive structured logging with correlation IDs for request tracing and debugging.

### 📁 Files to Create
```
backend/
├── middleware/
│   ├── requestLogger.js
│   └── correlationId.js
├── utils/
│   └── logger.js
└── tests/
    └── middleware/
        └── requestLogger.test.js
```

### 📝 Files to Modify
```
backend/
├── server.js
├── .env.example
└── package.json                 