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/domain/upload/models.go

@@ -0,0 +1,34 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+// Package upload 提供上传域的门面与类型重导出。
+package upload
+
+import (
+	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/models"
+)
+
+// UploadStatus 上传状态类型别名
+//
+//nolint:revive
+type UploadStatus = models.UploadStatus
+
+// UploadMetadata 上传元数据类型别名
+//
+//nolint:revive
+type UploadMetadata = models.UploadMetadata
+
+// Upload 上传实体类型别名
+type Upload = models.Upload
+
+// UploadStat 上传统计实体类型别名
+//
+//nolint:revive
+type UploadStat = models.UploadStat
+
+// 上传状态常量别名
+const (
+	UploadStatusPending = models.UploadStatusPending
+	UploadStatusUsed    = models.UploadStatusUsed
+	UploadStatusDeleted = models.UploadStatusDeleted
+)