GET /v1/orgs/:slug/billing/usage/sandbox — per-sandbox / per-day compute usage over a window (defaults to the current calendar month), computed on read from `sandbox_runs`.

Compute-on-read usage breakdown for an org over a window: per-(sandbox, day) quantity rows plus a directory of the sandboxes they reference. The frontend groups rows by sandbox (for the per-sandbox table) or by day (for the chart) and labels them via sandboxes.