Call Guest Link

Mint (or fetch) a guest link for a LIVE channel call.

The same row shape a native meeting uses — voice_share_token, role guest — but keyed by the CALL id rather than a persistent meeting code, because a phone call has no code and cannot be revived: the link is worth something only while the line is open, and /public/meetings/{token}/join refuses it with 409 once the call ends.

Idempotent by default: a second caller gets the link that already exists, so the button can be pressed twice without quietly invalidating the link a colleague is holding.