refactor(layout): relocate go.mod to backend/ and clean import paths to module root

- Relocated go.mod and go.sum into backend/ root directory
- Stripped redundant backend/ segments from all Go imports (github.com/Rain-kl/Wavelet/...)
- Unified Makefile, swagger, and build-test to execute in backend/ module context
- Ensured 100% build-test, code-check, format, and swagger pass

backend/plugins/domain/upload/util/utils.go

@@ -16,7 +16,7 @@ import (
 	"io"
 	"strings"
 
-	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/shared"
+	"github.com/Rain-kl/Wavelet/plugins/domain/upload/shared"
 	"github.com/deepteams/webp"
 	_ "golang.org/x/image/webp" // Register WebP decoder for image.Decode
 )