Behavior: Creates related content associations for each item in the provided list. This establishes bidirectional or unidirectional relationships between content items depending on the model configuration.

Authorization: Requires an authenticated user via the class-level [Authorize] attribute.

Side Effects: Creates metadata records linking content items as related to each other.

Error Scenarios: Returns 400 if the input is null or the items list is empty. Returns 404 if the items list is null.