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/main.go

@@ -4,7 +4,7 @@
 // Package main 是 Wavelet 平台的程序入口
 package main
 
-import "github.com/Rain-kl/Wavelet/backend/cmd"
+import "github.com/Rain-kl/Wavelet/cmd"
 
 // @title Wavelet API
 // @version 1.0.0