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/infra/database/postgres_logger.go

@@ -11,7 +11,7 @@ import (
 	"strings"
 	"time"
 
-	"github.com/Rain-kl/Wavelet/backend/pkg/logger"
+	"github.com/Rain-kl/Wavelet/pkg/logger"
 	"gorm.io/gorm"
 	gormLogger "gorm.io/gorm/logger"
 )