Relink Slack Identity

Forget the caller's own Slack mapping so the next send looks it up again.

Not admin-gated, and it needs no authorization code at all: company_id and user_id both come from the verified scope, so this route can only ever touch the caller's own row. That is also why it is a POST with no body.

cleared: true describes the state after the call, exactly like DELETE's disconnected: true, and not whether a row moved: clear_identity is idempotent, and "there was nothing cached" is a success — the next notification looks the user up fresh either way.

Why the row is deleted rather than nulled, and why there is no authorization code, both live in clear_identity.