Collateral-covered debt by collateral vault

Collateral-covered USD borrow exposure as a nested map keyed by borrow vault, then collateral vault. Underwater debt is capped at current attributable collateral value, so this endpoint is not gross open interest. The expensive account-level debt/collateral composition is worker-computed and stored in a latest projection table; the request path only reads that projection. Redis may cache this response, but PostgreSQL projection rows are the source of truth. Needs the risk chain capability; a chain without it answers 404 CHAIN_CAPABILITY_DISABLED.