Get the CHF exchange rate for a currency on a given date.

This operation remains available until <b>2027-08-14</b> and is removed after that date. Use <code>GET /core/v1/accounting/currencies/exchange-rate</code> instead.<br/><br/>Returns the Swiss National Bank reference rate that converts one unit of <code>from-currency-code</code> into CHF on <code>exchange-date</code>. When <code>from-currency-code</code> equals <code>CHF</code> (case-insensitive) the rate is always <code>1</code>. The exchange-rate table is global SNB data — the same response is returned for every tenant, so there are no tenant or company path parameters. If no rate is stored for the requested day, the downstream service transparently fetches the SNB feed for that date and persists the result; if the feed has no entry either, the most recent rate strictly before <code>exchange-date</code> is returned instead. The endpoint is a pure read for the caller — idempotent. Requires a bearer token whose principal carries the <code>ACCOUNTING</code> permission.