NOTIF-001: Notification Service — transactional emails for orders and account events

## Overview
Implement a notification service that sends transactional emails for key events: order confirmation, shipping update, password reset, and account registration.

## Acceptance Criteria
- [ ] Order confirmation email sent within 30s of order creation (ORD-001)
- [ ] Shipping update email sent when order status changes to 'Shipped'
- [ ] Password reset email (AUTH-003) routes through this service
- [ ] Registration verification email (AUTH-002) routes through this service
- [ ] All emails use a consistent branded template
- [ ] Unsubscribe link included in marketing emails

## Depends On
- #46 (ORD-001 — Order Service: triggers order emails)

Start by reading the dependency in #46 and trace the order-creation, shipping-status, password-reset, and registration-verification entry points it identifies. Confirm how each event reaches the notification service and define completion against the listed delivery timing, routing, branded-template, and unsubscribe acceptance criteria.

Written by the indexing model from the issue text.