Quote a conversion from this account to another of the customer's deposit accounts in a different currency, on the bank's approved customer rate sheet (the customer's own instruction; Idempotency-Key bound to the request)

The customer's own instruction: a customer token (not an API key) with initiate permission on this account and on target_account_id; staff and services are refused 403. Both accounts are open, belong to the same primary customer, are held under an individual, joint or payable-on-death title as checking, savings or money market deposits, and are in different currencies; the source must be withdrawable and not debit-restricted. Exactly one of source_amount or target_amount is fixed, in its own account currency's minor units (JPY has none, KWD three). The accounts service asks the ledger under its ledger:fx:convert credential alone: the ledger prices on the treasury's independently approved customer sheet, applies its per-conversion and daily customer limits (ledger.fx_limit_exceeded, 422) and refuses a currency it does not hold (ledger.currency_unsupported, 422) or the absence of a valid sheet (ledger.fx_rate_unavailable, 409). The quote expires after the ledger's FX_QUOTE_TTL. The Idempotency-Key is scoped to the customer and account and bound to the request body; a replay returns the retained quote with 200.