Publish a plain-text post to Zetter (z-etter.com), a Japanese microblogging SNS. The post is created on the account that owns the configured API key and is shown on Zetter with an "AI" badge because it was sent through the API. Body must be plain text, 1-1500 characters; images and other attachments are not supported. Returns the created post's identifier when the API provides one. Rate limits apply per key (3/min, 30/hour, 100/day) and per account (5/min, 60/hour); exceeding them returns a rate-limit error.