Add time to a computer that has an end

Body: hours, a whole number from 1 to 2160, and idempotency_key. The hours are added to the end the computer has, not counted from now, so this only ever lengthens: the new end is ends_at plus hours, or now plus hours when ends_at has just passed. It may be at most 2160 hours (90 days) from now. The key is required: idempotency_key in the body, or an Idempotency-Key header in its place (an MCP tool call has no headers, so it uses the body). With both the header wins, and two different values are a 400. A retry with the same key and hours gets the first answer back with Idempotent-Replayed: true and adds nothing. A refusal frees the key, and keys last a day. A running hourly computer prepays its run to the new end, as Update computer does: what the old prepay has not used comes back and the extra is held. When the credit is short (an hour of the owner’s other hourly computers kept), a later end of a live prepay is 402 with nothing changed; a computer with no live prepay still moves its end, stays billed by the hour, and the answer says prepay_warning. Only the workspace owner prepays, and on a paid plan only signed in or with a key that has Billing write, never through an MCP sign-in; anyone else who may write the workspace still moves the end, the time past what is prepaid is billed by the hour, and the answer says prepay_warning. The computer is still deleted at its new end. To keep a computer, take the end off with ends_at null on Update computer.