refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

backend/plugins/drivers/driver_asynq_worker/constants.go

@@ -0,0 +1,13 @@
+// Copyright 2025 linux.do
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package driver_asynq_worker
+
+// 任务队列名称
+const (
+	QueueDefault = "default"
+)
+
+// DefaultMaxRetry 任务默认最大重试次数
+const DefaultMaxRetry = 3