Create a new budget

BudgetScope is a closed oneof. Exactly one variant must be set. The six variants are ordered by enforcement precedence (most specific to most general) and mirror the BudgetScopeKind filter enum.

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.

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.

Optional threshold notifications. Ids are assigned by the server, so alerts[].id must be omitted here; supplying one is rejected.

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).