customerio-reference-architecture

# Customer.io Reference Architecture

## Overview
Enterprise-grade reference architecture for Customer.io integration with proper separation of concerns, reliability, and scalability.

## Architecture Diagram

```
                                    Customer.io
                                        |
                    +-------------------+-------------------+
                    |                   |                   |
              Track API            App API            Webhooks
                    |                   |                   |
                    v                   v                   v
            +-------+-------+   +-------+-------+   +-------+-------+
            |   Event Bus   |   |  Transactional |   | Webhook Handler|
            |   (Kafka)     |   |    Service     |   |   (Express)   |
            +-------+-------+   +-------+-------+   +-------+-------+
                    |                   |                   |
                    v                   v                   v
            +-------+-------+   +-------+-------+   +-------+-------+
            | CustomerIO    |   |    Email       |   |   Event       |
            | Worker        |   |    Templates   |   |   Processor   |
            +-------+-------+   +-------+-------+   +-------+-------+
                    |                   |                   |
                    +-------------------+-------------------+
                                        |
                                        v
                                +-------+-------+
                                |   Data Lake   |
                                |  (BigQuery)   |
                                +---------------+
```

## Instructions

### Step 1: Core Service Layer
```typescript
// src/services/customerio/index.ts
import { TrackClient, APIClient, RegionUS } from '@customerio/track';
import { EventEmitter } from 'events';

export interface CustomerIOConfig {
  trackSiteId: string;
  trackApiKey: s