Create a polling subscription over the change feed

Limited, machine-only release. Creates one feed bound to the authenticated customer and to the exact API key that made the request. No caller-supplied account, customer, credential, or key identifier is accepted, and the binding and the resource filter are immutable once created. Many feeds may share one key. The name is unique per account and key among live feeds: posting the same live name with the same resource returns the existing feed instead of creating a second one, and the same name with a different resource is refused. The response carries the start cursor captured from the feed high-water position at creation, so a client that stores it can poll from that point on. If the feed state cannot be read, the request fails and creates nothing.