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/task/rebuild_stats_test.go

@@ -0,0 +1,69 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package task
+
+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"
+)
+
+func TestRebuildUploadStatsHandler_Execute(t *testing.T) {
+	_, _, cleanup := testhelper.SetupTestEnvironment(t)
+	defer cleanup()
+
+	ctx := context.Background()
+	now := time.Now()
+
+	uploads := []models.Upload{
+		{
+			UserID: 1001, FileName: "a.jpg", FilePath: "uploads/a.jpg",
+			FileSize: 100, MimeType: "image/jpeg", Extension: "jpg", Hash: "hash-a",
+			Type: "pixez_mirror", Status: models.UploadStatusUsed, CreatedAt: now,
+		},
+		{
+			UserID: 1001, FileName: "b.png", FilePath: "uploads/b.png",
+			FileSize: 200, MimeType: "image/png", Extension: "png", Hash: "hash-b",
+			Type: "attachment", Status: models.UploadStatusUsed, CreatedAt: now,
+		},
+	}
+	for i := range uploads {
+		if err := database.DB(ctx).Create(&uploads[i]).Error; err != nil {
+			t.Fatalf("seed upload failed: %v", err)
+		}
+	}
+
+	// Corrupt stats to ensure rebuild recalculates from uploads.
+	if err := database.DB(ctx).Create(&models.UploadStat{
+		Dimension: models.UploadStatDimensionTotal,
+		StatKey:   "",
+		FileCount: 0,
+		FileSize:  0,
+	}).Error; err != nil {
+		t.Fatalf("seed broken total stat failed: %v", err)
+	}
+
+	handler := &RebuildUploadStatsHandler{}
+	result, err := handler.Execute(ctx, nil)
+	if err != nil {
+		t.Fatalf("Execute() error = %v", err)
+	}
+	if result == nil || result.Message == "" {
+		t.Fatalf("Execute() returned empty result: %+v", result)
+	}
+
+	var totalStat models.UploadStat
+	if err := database.DB(ctx).
+		Where("dimension = ? AND stat_key = ?", models.UploadStatDimensionTotal, "").
+		First(&totalStat).Error; err != nil {
+		t.Fatalf("load total stat failed: %v", err)
+	}
+	if totalStat.FileCount != 2 || totalStat.FileSize != 300 {
+		t.Fatalf("total stat = count %d size %d, want 2 / 300", totalStat.FileCount, totalStat.FileSize)
+	}
+}