List billing summary read models for the active scope

Returns the Billing Summary read model(s) for the active scope, backing the Billing summary page on route load. Each summary entry carries the right-rail metrics (all-time spend, month-to-date, projected monthly, hourly run rate, available credits, last-month total — all integers in ten-thousandths of the subscription currency's unit — plus the signed forecastDelta percentage vs last month) and the active service cards (per-service resource count, month-to-date spend and signed trend percentage). The collection normally contains a single row for the requested scope but is shaped as a standard paginated collection. Validation: scopeId must reference a tenant/subscription/project scope the caller can access — unknown scope maps to 404 NOT_FOUND; pagination parameters must be within bounds, else 422 VALIDATION_FAILED. No other inputs. Async & state: Synchronous read; no state change. The summary is recomputed server-side from the cost read model as ingestion progresses; no SSE frames are tied to this endpoint. Errors: 401 unauthenticated; 403 caller lacks a billing-capable role (owner|admin|contributor|reader|billing) on the scope; 404 unknown scopeId; 422 malformed query parameters; 500 read-model failure. Idempotency/Concurrency: Safe, cacheable; no idempotency key required.