Update Organization Budget

Replace a budget's label, figure and period.

Every field is optional and keyed on model_fields_set, matching the deployment-wide budget update's own: an omitted field is left alone, and an explicit null clears it, so sending max_budget: null takes a budget back to uncapped, which is what the dashboard's dialog does. The period pair is still mutually exclusive, and setting one does not clear the other, which is why :func:_require_single_period_source re-checks the resulting pair rather than the submitted one.

One of the organization's budgets, and how much of its own config names it.

Carries no spend rollup. BudgetResponse on the deployment surface sums users.spend over the gateway's users table, which is deployment-wide and has no tenancy column, so the same figure here would be a cross-tenant read. What an organization's own spend is, is a question for Usage.

ceiling_count is the organization-relevant fact instead: how many of its ceilings this budget currently holds, which is what makes a delete refuse.