Update a budget

BudgetLimits is the per-period spend and token ceiling. At least one of amount, token_limit, or RateLimit.requests_per_minute MUST be set on a Budget; that invariant is enforced by the handler.

RateLimit is the per-minute request ceiling. Enforced via atomic increment-first semantics in the enforcement middleware.

BudgetMatch carries the CEL expression that decides whether a budget applies to a request. Available variables: model, provider, model_id, api_key, identity, project, metadata (map), headers (map, lowercase keys). An empty expression always matches. Expressions are syntax-validated at write time.

Replaces the alert list wholesale. Omit to keep the current alerts. An entry with a known id is edited in place; one with no id is minted.

Budget is the canonical record stored in MongoDB budgets.entities. It replaces the embedded constraints.budget on api-keys and the legacy CONTACT-only budgets.configs collection (see ADR 0007).