feat(saas): add down migrations for all incremental schema changes (AUD3-DB-01)

- 16 down SQL files in migrations/down/ for each incremental migration
- db::run_down_migrations() executes rollback files in reverse order
- migrate_down CLI task: task=migrate_down timestamp=20260402
- Initial schema and seed data excluded (would be destructive)

crates/zclaw-saas/src/tasks/mod.rs

@@ -3,7 +3,7 @@
 //! 提供可手动执行的运维命令：
 //! - seed_admin — 创建管理员账号
 //! - cleanup_devices — 清理不活跃设备
-//! - migrate_schema — 手动触发 schema 迁移
+//! - migrate_down — 回滚指定迁移（timestamp=20260402 回滚所有 >= 该前缀的迁移）
 
 use std::collections::HashMap;
 use sqlx::PgPool;
@@ -27,6 +27,7 @@ pub fn builtin_tasks() -> Vec<Box<dyn Task>> {
     vec![
         Box::new(SeedAdminTask),
         Box::new(CleanupDevicesTask),
+        Box::new(MigrateDownTask),
     ]
 }
 
@@ -86,3 +87,18 @@ impl Task for CleanupDevicesTask {
         Ok(())
     }
 }
+
+/// 回滚数据库迁移
+struct MigrateDownTask;
+
+#[async_trait::async_trait]
+impl Task for MigrateDownTask {
+    fn name(&self) -> &str { "migrate_down" }
+    fn description(&self) -> &str { "回滚指定时间戳之后的迁移（timestamp=20260402 回滚 >= 该前缀的所有增量迁移）" }
+
+    async fn run(&self, db: &PgPool, args: &HashMap<String, String>) -> SaasResult<()> {
+        let timestamp = args.get("timestamp")
+            .ok_or_else(|| crate::error::SaasError::InvalidInput("Missing 'timestamp' argument (e.g. 20260402)".into()))?;
+        crate::db::run_down_migrations(db, timestamp).await
+    }
+}