sage posts don't update thread's `last_modified`

[bui]

I presume because bumpThread() is not called

solve by adding a last_modified column to the posts tables (possibly replace edited_at) and make the api builders use that column instead of bump

this should also solve the issue with 8archive not archiving past bump_limit, because as asagi (dumper) relies on the last_modified field, it doesn't update because 8chan doesn't change the field

https://github.com/ctrlcctrlv/infinity/blob/master/post.php#L951

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