Function apply_pending 

pub async fn apply_pending(
    pool: &Pool<Postgres>,
    migrations: &[Migration],
) -> Result<Vec<String>, CratestackError>

Apply every pending migration in the input slice in order. Each runs in its own transaction — Migration::up_pre then Migration::up, both inside it — and checksum drift aborts the whole apply (banks treat drift as a release-process failure for humans, not a silent overwrite).