Bind a fixed server IP to one of the account’s keys (starts pending)

The binding activates the first time that KEY calls the API from that IP — proof you control the address. From then on calls from the IP need no key and are authenticated, metered, scoped and billed as that key (X-Grundfast-Auth: ip-binding on the response). A presented key always wins; a wrong key is still a 401. Active bindings are immutable: delete and recreate to change one. Starter and up; the count is capped per plan.