Create API endpoint for assigning parent epic to child work items

Develop a backend API endpoint to allow linking child work items to a parent epic, ensuring hierarchical relationships are maintained.

**Story Points:** 3

Start by locating the existing backend API endpoints and the work-item and epic relationship models. Define the endpoint’s expected request and response from the project’s existing patterns, then verify that a child work item can be linked to a parent epic while maintaining the intended hierarchy.

Written by the indexing model from the issue text.