Go's encoding/json emits nil []T as null, not []. The initial
ClientPageResponseSchema and ClientHydrateSchema rejected null
inboundIds / summary.online / summary.depleted / etc., causing
[zod] warnings on every empty list.
Add nullableStringArray / nullableNumberArray helpers that accept
null and transform to [] so consuming code keeps seeing arrays.
Mark ClientRecord.traffic and .reverse nullable too (reverse is
explicitly null in MarshalJSON when storage is empty).