Publisher Add Show

Add a second (third, …) show to an ALREADY-AUTHENTICATED account.

Differences from the public onboard endpoint, all deliberate:

• the caller proves ownership of an existing tenant, so there is no email round-trip, no authorization re-prompt and no 24h review — the account is already opted in and its grant is inherited;
• the show is named from the feed, never from the URL's hostname;
• a slug collision is RESOLVED (name-2), not 409'd: the public endpoint's 409 protects a slug's original owner from a stranger, and inside an authenticated request there is no stranger;
• a feed already ingested by ANOTHER account is refused with the holder named, because the common cause is the author forgetting.