Get one scoped budget.
Successful Response
One scoped ceiling and its live counters.
Unlike /v1/budgets, the counters are the row's own: a scoped ceiling is enforced against current_spend + reserved_spend, so there is no rollup over users to compute.
Every limit, along with budget_duration_sec and reset_alignment, is read off the budget rather than stored here, and carried on the wire so a caller can render a ceiling without fetching every budget to resolve one id.