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/handler/file_management_test.go

@@ -29,7 +29,6 @@ func TestGetDistinctUploadTypes(t *testing.T) {
 		FileSize:      10,
 		MimeType:      "text/plain",
 		Extension:     "txt",
-		StorageDriver: "local",
 		Type:          "custom_type_xyz",
 		Status:        model.UploadStatusUsed,
 	}