Design a Rate Limiter (Practical)

Question

## Problem

Implement an API rate limiter (token bucket or sliding window) for GitHub's REST API, with per-user limits and rate-limit headers.

## Example
```
5000 req/hr per user; return remaining + reset headers
```

## Constraints
- Concurrent requests per user

## Approach
Token bucket per key; expose X-RateLimit headers. Discuss distributed enforcement.