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/downstream/plugins/custom_example/plugin.go

@@ -8,8 +8,8 @@ package custom_example
 import (
 	"net/http"
 
-	"github.com/Rain-kl/Wavelet/backend/core"
-	"github.com/Rain-kl/Wavelet/backend/core/contracts"
+	"github.com/Rain-kl/Wavelet/core"
+	"github.com/Rain-kl/Wavelet/core/contracts"
 	"github.com/gin-gonic/gin"
 )