Fix precision, scale of decimal columns on cart_item

The columns were originally created with precision=8 scale=5, which does not allow prices greater than 999.

This changes:
* `price` to the same as line_item.price (precision=8 scale=2)
* `ship_total` to the same as adjustment.amount (precision=10 scale=2)
* `amount` to precision=13 scale=5, which can fit the maximum allowable amount of either column at 5 decimal places of precision.