Check Domain Limit

Check whether admitting an address would break the domain signup rules.

In order: an excluded domain bans, an included domain admits, an existing account for the submitted address admits, else members are counted against SIGNUP_DOMAIN_LIMIT. That third rule makes an over-cap domain answer whether the address has an account; the IP rate limit is what bounds it.

Args: request: Incoming request, required by the rate limiter. body: Submitted address, as a full email or a bare domain.

Returns: Whether another signup is allowed, and whether the domain is banned.

Raises: HTTPException: 400 if the domain is empty or malformed.