Constant MIGRATION_DOWN_STUB 

pub const MIGRATION_DOWN_STUB: &str = r#"-- Migration: {%name%}
-- Direction: DOWN
-- Created: {%timestamp%}

-- 在此编写 down SQL
-- DROP TABLE IF EXISTS {%table_name%};
"#;

迁移 down SQL 模板（对齐 Phinx 风格）