Preview Shared Account Exposure

The blast-radius facts for sharing instance_id's account into a group.

READ ONLY. Mutates nothing, and returns exactly the body the 409 guard returns, so the "How should members connect?" screen renders the SERVER's member count, audit identity and blast-radius sentence instead of inventing them client-side or -- worse -- discovering them by firing a mutation it hopes will be refused.

That "hope it is refused" shape is not hypothetical: without a preview the only way to learn the facts is to attempt the share unconfirmed, which SUCCEEDS whenever the guard does not fire (a single-member group), silently performing the very operation the screen was about to ask permission for.

Authorization is the destination's own configure rung -- the same authority the move itself requires -- because the body names the destination's members and their identity ids. Answering it to a caller who could not perform the move would be a membership disclosure.