fix(storage): apply driver switch on save and repoint upload records

Saving storage settings now activates the selected driver immediately
instead of staging it until migration. When the driver changes, existing
upload rows are repointed to the new storage_driver so /f/{id} reads use
the correct backend. ForDriver can also open non-active drivers from the
saved multi-backend config.

.agent/skills/new-async-task/SKILL.md

@@ -43,7 +43,6 @@ description: "Wavelet 项目专用：新增或修改 Asynq 异步任务、后台
 - 失败返回 error，由任务框架处理状态和重试。
 - 不要吞掉关键错误。
 - 复杂 SQL 放到 `internal/model/` 或模块内的业务服务层（如 `internal/apps/<module>/service.go` 或 `logics.go`）。
-- 新增 Go 文件后检查许可证头，必要时运行 `make license`。
 
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