Daily reference exchange rates from the European Central Bank (via Frankfurter). 30+ major currencies. Pass base = 3-letter ISO 4217 currency (default USD), optional symbols = comma-separated target codes (default = all available), optional date = YYYY-MM-DD for historical rates (history back to 1999, business days only; omit for latest), optional amount to convert (default 1). Returns base, date, amount, and a map of currency code → rate.