fix: exchange rate uses core api endpoint

[talvasconcelos]

Fiat conversion rates are sent as tuple

[dni]

why?

[dni]

use endpoint from core instead. and remove the dead code

@api_router.get("/api/v1/rate/{currency}")
async def api_check_fiat_rate(currency: str) -> dict[str, float]:
    rate, price = await get_fiat_rate_satoshis(currency)
    return {"rate": rate, "price": price}