Task: Refactor Concrete Dependency to Injected Interface

// src/services/OrderService.ts
import { PostgresOrderRepository } from '../repositories/PostgresOrderRepository'

export class OrderService {
  private repo = new PostgresOrderRepository()

  async getOrderTotal(orderId: string): Promise<number> {
    const order = await this.repo.findById(orderId)
    if (!order) throw new Error(`Order ${orderId} not found`)
    return order.items.reduce((sum, item) => sum + item.price * item.quantity, 0)
  }

  async markShipped(orderId: string): Promise<void> {
    const order = await this.repo.findById(orderId)
    if (!order) throw new Error(`Order ${orderId} not found`)
    order.status = 'shipped'
    await this.repo.save(order)
  }
}