refactor(storage): drop per-upload storage_driver, use storage_config as single source

Remove w_uploads.storage_driver and route all read/write/delete paths through
storage.Active() backed by storage_config.driver. Block direct driver switches
when uploads exist; require migration task instead. Simplify migration to
cursor-based file_path iteration without per-row driver updates.

internal/apps/admin/system_config/errs.go

@@ -11,4 +11,5 @@ const (
 	ConfigKeyRequired    = "配置键不能为空"
 	ConfigValueRequired  = "配置值不能为空"
 	ConfigKeyExists      = "配置键已存在"
+	StorageDriverSwitchRequiresMigration = "存在存量文件，请通过存储迁移任务切换存储引擎"
 )