Added: expires_at field in Reward schema

We've added a new expires_at attribute to the reward schema. This field returns the datetime when a reward expires, allowing you to display expiration dates to recipients.

The field is returned as an ISO 8601 timestamp (e.g., "2025-03-15T23:59:59Z") and will be null for rewards that don't expire.

Example response:

{
  "id": "ABCD1234",
  "order_id": "WXYZ5678",
  "created_at": "2025-01-15T10:30:00Z",
  "expires_at": "2025-07-15T09:59:59Z",
  "value": {
    "denomination": 50.0,
    "currency_code": "USD"
  },
  ...
}

This field is now included in responses from:

GET /rewards
GET /rewards/[id]
POST /orders (in the rewards array)
GET /orders/[id] (in the rewards array)