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/storage_ops.go

@@ -0,0 +1,31 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package storage
+
+import (
+	"context"
+
+	"github.com/Rain-kl/Wavelet/backend/pkg/logger"
+	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/models"
+	"github.com/Rain-kl/Wavelet/backend/plugins/infra/storage/objectstore"
+)
+
+// ReadOnly checks if the storage system is in read-only maintenance mode.
+func ReadOnly(ctx context.Context) bool {
+	state := LoadMigrationAccessState(ctx)
+	if state.LoadErr != nil {
+		logger.ErrorF(ctx, "读取存储维护状态失败: %v", state.LoadErr)
+		return true
+	}
+	return state.ReadOnly
+}
+
+// OpenStoredObject opens a stored upload object from the active storage backend.
+func OpenStoredObject(ctx context.Context, upload *models.Upload) (*objectstore.Object, error) {
+	_, backend, err := objectstore.Active(ctx)
+	if err != nil {
+		return nil, err
+	}
+	return backend.Get(ctx, upload.FilePath)
+}