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/models.go

@@ -5,7 +5,7 @@
 package upload
 
 import (
-	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/models"
+	"github.com/Rain-kl/Wavelet/plugins/domain/upload/models"
 )
 
 // UploadStatus 上传状态类型别名