Revokes the refresh tokens for an OAuth client family (e.g. an MCP client) so it can no longer mint access tokens. Pass family_id to revoke ONE grant — a client the user authorized twice has two, and each is its own row in GET /connections. Omit it to disconnect the client entirely, which is what frees a connection-cap slot. vault_id is the older, coarser narrowing and is ignored when family_id is given. Session-auth only.