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/util/media.go

@@ -0,0 +1,51 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package util
+
+import (
+	"strings"
+
+	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/shared"
+)
+
+// IsImageExtension reports whether ext is a common image format.
+func IsImageExtension(ext string) bool {
+	for _, imgExt := range []string{"jpg", "jpeg", "png", "webp", "gif"} {
+		if ext == imgExt {
+			return true
+		}
+	}
+	return false
+}
+
+// IsArchiveExtension reports whether ext is a common archive format.
+func IsArchiveExtension(ext string) bool {
+	for _, e := range []string{"zip", "rar", "7z", "tar", "gz", "tgz", "bz2", "xz"} {
+		if ext == e {
+			return true
+		}
+	}
+	return false
+}
+
+// IsDocumentExtension reports whether ext is a common document format.
+func IsDocumentExtension(ext string) bool {
+	for _, e := range []string{"pdf", "doc", "docx", "xls", "xlsx", "ppt", "pptx", "txt", "md", "csv", "json", "yaml", "yml", "xml"} {
+		if ext == e {
+			return true
+		}
+	}
+	return false
+}
+
+// NormalizeImageQuality normalizes the requested image quality query parameter.
+func NormalizeImageQuality(quality string) string {
+	q := strings.TrimSpace(strings.ToLower(quality))
+	switch q {
+	case shared.ImageQualityLow, shared.ImageQualityMedium, shared.ImageQualityHigh:
+		return q
+	default:
+		return shared.ImageQualityOrigin
+	}
+}