List billable resource subscriptions with search, filter, sort and pagination

Returns the per-resource recurring subscriptions (billable resources) in the active tenant/subscription/project scope, as rendered by the Subscriptions view: one row per billable resource with its billing cycle, unit price and usage-to-date in ten-thousandths of the currency unit, quantity and next invoice date. This single operation serves both the plain route-load list and the search/filter/sort/paginate variant (billing.billing_subscription.search_filter_sort): q matches free text over name and service; cycle narrows to the Hourly/Monthly counter filters; sort accepts service | name | unitPrice | usageToDate with order asc|desc. KPI rollups (monthly run rate, usage this period) are computed client-side from this data. Validation: caller must hold at least the reader or billing role in the scope (else 403); scopeId must resolve to a visible tenant/subscription/project (else 404 NOT_FOUND); page >= 1, pageSize 1..200, cycle/order must be enum members and sort a supported field — violations map to 422 VALIDATION_FAILED naming the offending field. Async & state: Synchronous read; no state change. No operation is created and no resource.updated SSE frames are emitted. Errors: 401 missing/expired session; 403 role lacks billing read access in this scope; 404 unknown scopeId; 422 malformed pagination, sort or enum filter values; 500 internal. Idempotency/Concurrency: Safe, cacheable; no idempotency key required. The response is always the { data, meta } envelope — never a naked array.