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/upload/task/cleanup.go

@@ -84,11 +84,7 @@ func (h *SystemCleanupHandler) Execute(ctx context.Context, _ []byte) (*task.Tas
 					return err
 				}
 
-				driver := storage.Driver(u.StorageDriver)
-				if driver == "" {
-					driver = storage.DriverLocal
-				}
-				backend, err := storage.ForDriver(ctx, driver)
+				_, backend, err := storage.Active(ctx)
 				if err != nil {
 					return err
 				}