Create one SWAP_ONLY standing order per eligible (i.e. a crypto account that is enriched) non-EUR account in a wallet under a single SCA challenge. Today only direction=CRYPTO_TO_FIAT is supported: every crypto account in the wallet will auto-swap its deposits into the wallet's EUR account, with no external SEPA payout. FIAT_TO_CRYPTO is intentionally not supported here — use the per-account /create endpoint, since a single EUR source has many possible crypto destinations.

The endpoint is all-or-nothing: if any candidate account already has an active SO, the call rejects up front. On success, the SCA challenge returned covers the entire batch — confirm it once via the standard /confirm endpoint and one SO row is materialized per account.