Availability notifications let customers subscribe to back-in-stock alerts for out-of-stock variants. When the variant is restocked, the customer receives an email notification.

## Create Availability Notification

```
POST /api/v1/availability-notifications
```

Registers a back-in-stock alert for a product variant. Idempotent — if the same email is already subscribed for the same variant, the response indicates `already_subscribed` without creating a duplicate.

### Request Body

| Field | Type | Required | Description |
|-------|------|----------|-------------|
| `email` | `string` | Yes | Customer email to notify on restock |
| `name` | `string` | No | Customer display name |
| `productVariantId` | `string` (UUID) | Yes | Variant to watch for back-in-stock |

```bash
curl -X POST \
  -H "Authorization: Bearer your_api_key" \
  -H "Content-Type: application/json" \
  -d '{"email": "customer@example.com", "productVariantId": "0191abc0-1234-7def-8000-000000000001"}' \
  https://your-store.yns.store/api/v1/availability-notifications
```

### Response (201) — New Subscription

```json
{
  "status": "subscribed"
}
```

### Response (200) — Already Subscribed

```json
{
  "status": "already_subscribed"
}
```

### Error Responses

| Status | Description |
|--------|-------------|
| `400` | Invalid request data (missing email, invalid UUID, etc.) |
| `404` | Variant not found or does not belong to this store |