Remove amount column from statement_rows [#86776]

The amount was only being set on StatementRow creation, which meant it missed
updates to OrderDetail#total. With this change, StatementRow#amount gets its
value directly from OrderDetail#total.
Implement reverse migration [#86776]
Clean up specs [#86776]
Make down migration Oracle-friendly [#86776]
Merge pull request #144 from tablexi/86776-po-statement-total

Remove amount column from statement_rows [#86776]