refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

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

@@ -0,0 +1,103 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package stats
+
+import (
+	"context"
+	"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"
+	"gorm.io/gorm"
+)
+
+func TestApplyUploadStatsDeltaTxWithinTransaction(t *testing.T) {
+	_, _, cleanup := testhelper.SetupTestEnvironment(t)
+	defer cleanup()
+	ctx := context.Background()
+
+	upload := &models.Upload{
+		ID:        42002,
+		FileSize:  256,
+		MimeType:  "image/jpeg",
+		Extension: "jpg",
+		Type:      "avatar",
+		Status:    models.UploadStatusUsed,
+		CreatedAt: time.Now(),
+	}
+
+	if err := database.DB(ctx).Transaction(func(tx *gorm.DB) error {
+		return ApplyUploadStatsDeltaTx(tx, upload, 1)
+	}); err != nil {
+		t.Fatalf("ApplyUploadStatsDeltaTx returned error: %v", err)
+	}
+
+	stats, err := loadUploadStats(ctx)
+	if err != nil {
+		t.Fatalf("loadUploadStats returned error: %v", err)
+	}
+	if stats.TotalCount != 1 || stats.TotalSize != 256 {
+		t.Fatalf("unexpected total stats: count=%d size=%d", stats.TotalCount, stats.TotalSize)
+	}
+}
+
+func TestApplyUploadStatsAddAndRemove(t *testing.T) {
+	_, _, cleanup := testhelper.SetupTestEnvironment(t)
+	defer cleanup()
+	ctx := context.Background()
+
+	upload := &models.Upload{
+		ID:        42001,
+		FileSize:  128,
+		MimeType:  "image/png",
+		Extension: "png",
+		Type:      "avatar",
+		Status:    models.UploadStatusUsed,
+		CreatedAt: time.Now(),
+	}
+	if err := ApplyUploadStatsAdd(ctx, upload); err != nil {
+		t.Fatalf("ApplyUploadStatsAdd returned error: %v", err)
+	}
+
+	stats, err := loadUploadStats(ctx)
+	if err != nil {
+		t.Fatalf("loadUploadStats returned error: %v", err)
+	}
+	if stats.TotalCount != 1 || stats.TotalSize != 128 {
+		t.Fatalf("unexpected total stats: count=%d size=%d", stats.TotalCount, stats.TotalSize)
+	}
+
+	if err := ApplyUploadStatsRemove(ctx, upload); err != nil {
+		t.Fatalf("ApplyUploadStatsRemove returned error: %v", err)
+	}
+
+	stats, err = loadUploadStats(ctx)
+	if err != nil {
+		t.Fatalf("loadUploadStats after remove returned error: %v", err)
+	}
+	if stats.TotalCount != 0 || stats.TotalSize != 0 {
+		t.Fatalf("expected zeroed total stats, got count=%d size=%d", stats.TotalCount, stats.TotalSize)
+	}
+}
+
+type uploadStatsSnapshot struct {
+	TotalCount int64
+	TotalSize  int64
+}
+
+func loadUploadStats(ctx context.Context) (uploadStatsSnapshot, error) {
+	var rows []models.UploadStat
+	if err := database.DB(ctx).Where("dimension = ?", models.UploadStatDimensionTotal).Find(&rows).Error; err != nil {
+		return uploadStatsSnapshot{}, err
+	}
+	if len(rows) == 0 {
+		return uploadStatsSnapshot{}, nil
+	}
+	return uploadStatsSnapshot{
+		TotalCount: rows[0].FileCount,
+		TotalSize:  rows[0].FileSize,
+	}, nil
+}