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/auth/repository.go

@@ -7,7 +7,7 @@ package auth
 import (
 	"context"
 
-	db "github.com/Rain-kl/Wavelet/backend/plugins/infra/database"
+	db "github.com/Rain-kl/Wavelet/plugins/infra/database"
 )
 
 // GetAuthSourceByID 根据 ID 获取认证源