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/storage/migration.go

@@ -0,0 +1,80 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package storage
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"strings"
+
+	"github.com/Rain-kl/Wavelet/backend/plugins/drivers/driver_asynq_worker"
+	"github.com/Rain-kl/Wavelet/backend/plugins/infra/storage/objectstore"
+)
+
+// StorageMigrationTask is the Asynq task name for storage migration.
+const StorageMigrationTask = "storage:migrate"
+
+// LatestMigrationExecution returns the most recent storage migration task execution.
+func LatestMigrationExecution(ctx context.Context) (*driver_asynq_worker.TaskExecution, bool, error) {
+	return driver_asynq_worker.GetLatestTaskExecutionByTaskType(ctx, StorageMigrationTask)
+}
+
+// ParseMigrationTargetConfig parses and validates a storage migration target payload.
+func ParseMigrationTargetConfig(ctx context.Context, payload []byte) (objectstore.Config, error) {
+	if strings.TrimSpace(string(payload)) == "" {
+		return objectstore.Config{}, errors.New("storage migration target payload is required")
+	}
+
+	var raw struct {
+		Target json.RawMessage `json:"target"`
+	}
+	if err := json.Unmarshal(payload, &raw); err != nil {
+		return objectstore.Config{}, fmt.Errorf("parse storage migration payload envelope: %w", err)
+	}
+
+	if len(raw.Target) == 0 {
+		return objectstore.Config{}, errors.New("storage migration target payload is required")
+	}
+
+	var targetBytes []byte
+	var targetStr string
+	if err := json.Unmarshal(raw.Target, &targetStr); err == nil {
+		targetBytes = []byte(targetStr)
+	} else {
+		targetBytes = raw.Target
+	}
+
+	var target objectstore.Config
+	if err := json.Unmarshal(targetBytes, &target); err != nil {
+		return objectstore.Config{}, fmt.Errorf("parse target storage config: %w", err)
+	}
+
+	current, err := objectstore.LoadConfig(ctx)
+	if err != nil {
+		return objectstore.Config{}, fmt.Errorf("load active storage config: %w", err)
+	}
+	target = objectstore.MergeMaskedSecrets(target, current)
+	if err := objectstore.ValidateConfig(target); err != nil {
+		return objectstore.Config{}, fmt.Errorf("validate target storage config: %w", err)
+	}
+	return target, nil
+}
+
+// NormalizeMigrationPayload validates and normalizes a storage migration payload.
+func NormalizeMigrationPayload(ctx context.Context, payload []byte) ([]byte, objectstore.Config, error) {
+	target, err := ParseMigrationTargetConfig(ctx, payload)
+	if err != nil {
+		return nil, objectstore.Config{}, err
+	}
+	type storageMigrationPayload struct {
+		Target objectstore.Config `json:"target"`
+	}
+	normalized, err := json.Marshal(storageMigrationPayload{Target: target})
+	if err != nil {
+		return nil, objectstore.Config{}, fmt.Errorf("marshal storage migration payload: %w", err)
+	}
+	return normalized, target, nil
+}