Build a Rate Limiter With Delivery Tracking

Question

## Problem

Build a per-account **rate limiter** for an outbound messaging API, plus track delivery status (queued/sent/delivered/failed) per message.

## Example
```
limit 5 msg/sec/account; 6th in the same second is throttled or queued
```

## Constraints
- Many accounts; concurrent sends

## What Twilio looks for
Practical, modular code (not a puzzle). Token bucket per account; clean status state machine; how you'd test it.