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/errs.go

@@ -0,0 +1,17 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package driver_asynq_worker
+
+const (
+	errUnknownTaskType            = "未知的任务类型: %s"
+	errCreateTaskExecutionFailed  = "创建任务执行记录失败: %w"
+	errTaskEnqueueFailed          = "任务入队失败: %w"
+	errTaskExecutionNotFound      = "任务执行记录不存在: %w"
+	errRetryOnlyFailedTask        = "只有失败的任务才能重试，当前状态: %s"
+	errTaskNotRetryable           = "该任务不支持重试"
+	errTaskMaxRetryExceeded       = "已达到最大重试次数 %d"
+	errCreateRetryExecutionFailed = "创建重试任务执行记录失败: %w"
+	errRetryTaskEnqueueFailed     = "重试任务入队失败: %w"
+	errUnregisteredTaskHandler    = "未注册的任务处理器: %s"
+)