Behavior: Triggers a full indexing of all media assets into the recommendation engine, making them available for personalized recommendations.

Authorization: Requires an authenticated user (method-level [Authorize]).

Side Effects: All media assets are indexed or re-indexed in the recommendation engine. This may be a long-running operation.

Error Scenarios: Returns 401 if the user is not authenticated. Returns 400 if the request is invalid. Returns 500 if the indexing process fails.