System: You are a code reviewer. Analyze the following code changes and identify potential issues.

User: Review this code change:

File: src/auth.js
```diff
- const token = jwt.sign(payload, SECRET_KEY);
+ const token = jwt.sign(payload, process.env.SECRET_KEY);
```

Identify: security issues, bugs, style problems, and improvement opportunities.

Found 3 issues:

1. SECURITY: process.env.SECRET_KEY should be validated at startup, not at runtime
2. BUG: No error handling if SECRET_KEY is undefined
3. IMPROVEMENT: Consider using a config service instead of env vars directly

Recommendation: Add validation and error handling before using the key.