Get usage and quota

What each pool has used, its limit, when it refills, and what has recently been refused.

This endpoint does NOT consume a metered request, and needs no particular scope beyond a valid key. Both are deliberate: the callers who most need to read their quota are the ones who have run out, and a key holding only write scopes still has to be able to discover why its writes started failing.

Use it to pace work before a large batch, and to interpret a 429 after one.