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/stats/category.go

@@ -7,7 +7,7 @@ package stats
 import (
 	"strings"
 
-	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/util"
+	"github.com/Rain-kl/Wavelet/plugins/domain/upload/util"
 )
 
 const (

backend/plugins/domain/upload/stats/stats_counter.go

@@ -7,9 +7,9 @@ import (
 	"context"
 	"time"
 
-	"github.com/Rain-kl/Wavelet/backend/pkg/logger"
-	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/models"
-	database "github.com/Rain-kl/Wavelet/backend/plugins/infra/database"
+	"github.com/Rain-kl/Wavelet/pkg/logger"
+	"github.com/Rain-kl/Wavelet/plugins/domain/upload/models"
+	database "github.com/Rain-kl/Wavelet/plugins/infra/database"
 	"gorm.io/gorm"
 	"gorm.io/gorm/clause"
 )

backend/plugins/domain/upload/stats/stats_counter_test.go

@@ -8,9 +8,9 @@ import (
 	"testing"
 	"time"
 
-	"github.com/Rain-kl/Wavelet/backend/pkg/testhelper"
-	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/models"
-	database "github.com/Rain-kl/Wavelet/backend/plugins/infra/database"
+	"github.com/Rain-kl/Wavelet/pkg/testhelper"
+	"github.com/Rain-kl/Wavelet/plugins/domain/upload/models"
+	database "github.com/Rain-kl/Wavelet/plugins/infra/database"
 	"gorm.io/gorm"
 )