Get the authenticated user's prepaid trading-fee configuration: the
fee-receiving party id and the configured fee asset.

Total fees consumed and deposited (total_fees_consumed,
total_fees_deposited) and effective maker/taker fee rates are not
included here — read them from /auth/me (total_fees_consumed,
total_fees_deposited, maker_fees/taker_fees), which resolves each
side from an active referral code, then per-user overrides (custom
market-maker configs), then the platform default rates.

Prepaid fee balances are not included here either — read them from
GET /api/trading/balances (fee_balances).

fee_enabled is the platform-wide fee-collection toggle. When
false, trades are not charged fees and prepaid fee balances are
not enforced.

fee_party identifies the destination party for prepaid-fee top-up
transfers; it is "" when not configured.

fee_asset is the single configured prepaid-fee asset, returned as
its instrument_id, symbol, and admin (the instrument's issuer
party); each is "" when not configured.