[Mint] Refactor set_melt_quote_paid

[callebtc]

In various parts of the mint code base, we repeat these steps to set a melt quote as paid. This should be refactored into its own method ledger.set_melt_quote_paid(fee: Optional[Amount] = None) that should be called instead.

Note: set_melt_quote_paid should check if the unit for the fee: Amount argument is the same as quote.unit and error otherwise.

Example code:

quote.paid = True
if payment.fee:
    quote.fee_paid = payment.fee.to(Unit[quote.unit]).amount
quote.proof = payment.preimage or ""
await self.crud.update_melt_quote(quote=quote, db=self.db)