Create a sandbox account and mint its first API key

Create a sandbox account and return {account_id, api_key, tier, limits, billing, docs, email, email_confirmation}; api_key is shown once. ClipZyn emails a confirmation link to the returned email (the normalized address) and the key answers 403 email_unconfirmed until a person opens it. This call costs nothing quota-wise and must come before authenticated operations. Send one Idempotency-Key per signup intent. Recover from account_exists by asking the owner to use the web key page, from validation_error by fixing the named field, and from rate_limited or signup_capacity by waiting retry_after_sec before retrying.