Delete several files from S3 in one request

Delete many files in ONE request. Body: {"keys": [...]}.

Batched server-side rather than looped client-side: the single-key routes are async def running blocking boto3, so N concurrent client calls would put N blocking round-trips on the shared event loop.

Returns {"deleted": n, "results": [{"key", "status", "erro"?}, ...]} — status is deleted | invalid | error. S3 reports success for a key that never existed, so deleted means "no longer present".