Lock previewed FX rate with `quote_id` on payouts

What's new — Preview endpoints now return a quote_id (and quote_expires_at) when called with create_quote: true. Pass that quote_id on the subsequent payout request to settle at the previewed amount, fees, and FX rate. Quotes expire after 10 minutes.

Impact — Optional. Without quote_id, payouts continue to be priced at execution time using the live rate.

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# Step 1 — preview with create_quote: true
curl -X POST .../v1/virtual-accounts/{id}/payout/preview \
  -d '{ "amount": "100.00", "currency": "USD", "create_quote": true }'
# → { "quote_id": "quo_abc", "quote_expires_at": "...", ... }

# Step 2 — settle at the locked rate
curl -X POST .../v1/virtual-accounts/{id}/payout \
  -d '{ "amount": "100.00", "currency": "USD", "quote_id": "quo_abc" }'

Same flow works on POST /v1/payouts.

Reference — POST /v1/virtual-accounts/{id}/payout/preview, POST /v1/payouts