List backup plans in the active scope

Returns the backup PLANS of the active scope. A plan is a reusable policy — a schedule and a retention window — that any number of volumes are assigned to, so this list is short and stable where the older per-volume plan list grew one row per protected volume. Each row carries assignedVolumeCount, the last run's time and outcome, and the next run time, which is what the plans table renders without a second call. Validation: scopeId must resolve to a scope the caller can access (unknown or soft-deleted → 404 NOT_FOUND); page ≥ 1 and pageSize within server bounds → 422. RBAC: any scope role may read. Async & state: Synchronous read; no state change. Errors: 401 unauthenticated; 403 no role on the scope; 404 unknown scopeId; 422 malformed pagination; 500 unexpected backend failure. Idempotency/Concurrency: Safe and cacheable; no idempotency key.