Update Rates (auto-sync)

Refreshes every non-base currency's exchange rate from the store's configured external rate provider — the API equivalent of the Update Rates button on the Exchange Rates screen.

• Takes no request body.
• On success returns { success, updated, message }, where updated is the number of exchange-rate rows present after the sync.
• If the external provider fails (for example a missing or invalid API key, or a network error), the endpoint returns 422 with the provider's own error message.
• The provider is configured server-side; this endpoint does not choose or accept a provider.

Permission: settings.exchange_rates.edit.