重构: 拆migrations SQL迁移(strategy核心库)

- 新建 df-storage/migrations_helpers.rs(103行): column_exists纯查询fn + 4测试
- migrations.rs 659→541: use column_exists + 删原fn+测试, run/migrate_v1-v19主入口保留
- lib.rs: pub mod migrations_helpers
主代grep核验(不cargo避executor-split df-workflow中间态): migrations.rs use+column_exists 16处调用 + lib.rs pub mod印证; agent df-storage独立自验cargo 0+test 35+11
strategy: 核心库自底向上, 纯查询函数抽离(PRAGMA无副作用, SQL行为不变), pub(crate)零外部暴露扩大
git add指定(df-storage/*)

crates/df-storage/src/lib.rs

@@ -3,5 +3,6 @@
 pub mod crud;
 pub mod db;
 pub mod migrations;
+pub mod migrations_helpers;
 pub mod models;
 pub mod secret;