Add precision validation for Moonpay call

[ok300]

The Moonpay API defines BTC amounts as having precision = 5, so only 5 decimals are considered. This means sat amounts must be mutiples of 1_000.

When this is not the case, the Moonpay URL shows an ambiguous error:

"Invalid queries, check 'errors' property for more info."

which, when checked, shows another ambiguous

quoteCurrencyAmount must be a decimal number

thrown by the getbuyquote endpoint. Here, quoteCurrencyAmount is the sat amount expressed as BTC (e.g. 0.0005).