Set the account's monthly overage budget

Sets budget_usd_cents — an integer number of USD cents, at most 500000 ($5,000/month; the cap bounds the blast radius of a compromised session). Setting 0 turns overage off.

PAID PLANS ONLY: free never bills overage (its caps refuse) and enterprise is uncapped with no overage billing, so both are refused 403 plan_required — the error's details carry plan and available_on so a client can render an upgrade prompt.

Overage is billed per meter at the plan's published rates (overage on GET /v2/plans), added to the regular subscription invoice, and clamped to this budget.