Rotate the project's Swift Temp-URL key

Replaces the account's Temp-URL-Key (slot 1) with a new random value. The new key is never returned. The body is optional; an empty body means mode: revoke.

• revoke (default): also removes Temp-URL-Key-2 and every container-level key. Every Temp-URL signed with an old key stops working at once -- including URLs other members handed out -- and the pending rotation advice is cleared.
• graceful: the current slot-1 key moves to Temp-URL-Key-2 and keeps working -- any key already in Temp-URL-Key-2 is replaced by it; container keys are untouched and the rotation advice is NOT cleared (the old key is still valid). Exactly one previous key survives, so a second graceful rotation retires it. Key slots: the platform signs console upload/download links only with slot 1 and reuses whatever slot-1 key the account holds. Slot 2 (Temp-URL-Key-2) is replaced by EVERY rotation (revoke removes it; graceful overwrites it with the previous slot-1 key). The only key the platform never reads, writes or rotates is a container-level key (X-Container-Meta-Temp-URL-Key) -- except a revoke rotation, which removes every key including container keys. The console's own links keep working after either mode (they are signed with the current key on demand). Errors: 401 unauthenticated; 403 caller lacks manage access to the scope's storage; 404 unknown scopeId; 409 the project is not provisioned, or 409 details.reason=project_suspended while the project is suspended; 422 unknown mode; 502/503 the object store refused or is unavailable.