Validate price maximum.

Price is constrained to be precision: 8, scale: 2 at the database level, but
that validation isn't reflected in the model.

This will lead to errors when prices are 1,000,000 or above, which is a lot in
USD, but 1 million Vietnamese Dongs is less than $50 USD.
I think we could put this in a method `'999999.99'.to_d` just to make it easier in case some stores decide to put a higher value? not sure if theres a way to catch the precision via active record?