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/shared/constants.go

@@ -0,0 +1,25 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package shared
+
+// Upload size, path, media quality, and cache constants shared across subpackages.
+const (
+	MaxUploadSize           = 32 * 1024 * 1024 // 32MB
+	DetectContentBytes      = 512              // http.DetectContentType 需要的最小字节数
+	UploadDirPerm           = 0755             // 上传目录权限
+	UploadFilePerm          = 0644             // 上传文件权限
+	ImageQualityLow         = "low"
+	ImageQualityMedium      = "medium"
+	ImageQualityHigh        = "high"
+	ImageQualityOrigin      = "origin"
+	DefaultPublicUploadType = "avatar"
+	FileStatsTrendDays      = 7
+	MaxS3KeyLength          = 1024
+	AccessCacheTTL          = 5 // seconds; multiplied by time.Second at use site
+
+	UploadStatDimensionTotal    = "total"
+	UploadStatDimensionType     = "type"
+	UploadStatDimensionCategory = "category"
+	UploadStatDimensionTrend    = "trend"
+)