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/stats/category.go

@@ -0,0 +1,43 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+// Package stats maintains incremental upload statistics and aggregations.
+package stats
+
+import (
+	"strings"
+
+	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/util"
+)
+
+const (
+	catImage    = "图片"
+	catVideo    = "视频"
+	catAudio    = "音频"
+	catDocument = "文档"
+	catArchive  = "压缩包"
+	catOther    = "其他"
+)
+
+// GetFileCategory classifies a file by mime type and extension.
+func GetFileCategory(mimeType, ext string) string {
+	mimeType = strings.ToLower(mimeType)
+	ext = strings.ToLower(ext)
+
+	if strings.HasPrefix(mimeType, "image/") || util.IsImageExtension(ext) {
+		return catImage
+	}
+	if strings.HasPrefix(mimeType, "video/") {
+		return catVideo
+	}
+	if strings.HasPrefix(mimeType, "audio/") {
+		return catAudio
+	}
+	if util.IsArchiveExtension(ext) || strings.Contains(mimeType, "zip") || strings.Contains(mimeType, "tar") || strings.Contains(mimeType, "gzip") {
+		return catArchive
+	}
+	if util.IsDocumentExtension(ext) || strings.HasPrefix(mimeType, "text/") || mimeType == "application/pdf" {
+		return catDocument
+	}
+	return catOther
+}