ndcg

Anyone can call a rerank endpoint. The word doing the work in this question is 'prove': you have to implement DCG, ideal DCG, and nDCG@k from graded labels and report the before-and-after. Plus the ceiling nobody mentions until it bites them in production.

Ranking is not classification, so accuracy is the wrong instrument. What you need to show is matching the rank-aware metric to the task: one right answer versus many, binary versus graded relevance. Here is the answer.