Funded balance + leverage for a customer's reward inventory.
Returns null when no inventory wallet is on record for the customer (not
provisioned). A known wallet whose on-chain balance can't be fetched is
not null: balance.amount is null and leverage.status is UNAVAILABLE,
so callers distinguish "no inventory" from "couldn't read it right now".
Read-only view of a customer's reward inventory: funded on-chain balance and the leverage of owed rewards against it.
Reads the inventory wallet address from the persisted
reward_inventorytable — it never touches the Doppler-backed keypair — so it is safe to wire into customer-facing apps. The address is provisioned by RewardInventoryAdminService (backfill + the signing path).