how do ceph s3 user and subuser permissions work together?

Your bucket policy should be like this:

{
  "Version": "2012-10-17",
  "Statement": [{
    "Effect": "Deny",
    "Principal": {"AWS": ["arn:aws:iam:::user/s3test:backup"]},
    "Action": "s3:PutObject",
    "Resource": [
      "arn:aws:s3:::hedgehogs/*"
    ]
  }]
}