[feat: backend] Add a fulfillment reference to the Have schema

[therealjustinbiele]

Description

If a Have is getting acquired by someone, we would say that the Have is fulfilled.

Acceptance Criteria

• Update the Have data model to include a fulfilled reference
• Update the getHaves controller to accommodate a fulfilled: boolean query parameter
• Alternatively, you can create another endpoint with another controller function to return fulfilled items / unfulfilled items