Admin team management — paginated workspace list with search (admin-gated)

List every workspace, newest first, with per-team activity counts.

Per row: id, name, plan, owner (id + email + plan), workspace_active, member_count, pending_invitations, seats_reserved vs max_seats, agent_count, run_count, created_at, last_activity.

Auth: anonymous → 401, non-admin → 403, admin → 200. Flag-off → 404 (rule #4). Pagination caps at 100 (rule #9).