Get plan usage summary

Retrieve the organization's plan minutes consumption for the current billing period — the same numbers shown by the dashboard usage bar.

Returns the minutes included in the plan, the minutes already used,
the minutes remaining from the quota, the extra minutes consumed beyond
the quota, and the billing period bounds. Internal credit rates and
pricing details are intentionally omitted.

Authentication:
- API key with READ or READ_WRITE permission, or
- OAuth access token granted the `usage:read` scope