Batch quote fetch by explicit ticker list

Fetch real-time quotes for up to 500 tickers in a single call.

Tickers are deduped and uppercased. Tickers with no quote available map to null (rather than being absent from the response object). Requests are batched and run in parallel; results are cached for 60s.

Credit cost: max(1, ceil(N / 10)) where N is the number of tickers in the request.