重构: 存储传输层集成 rclone 替代自研实现

将 8 种存储后端（本地磁盘、S3、WebDAV、Google Drive、FTP、阿里云 OSS、
腾讯云 COS、七牛 Kodo）的底层传输从 4 个独立 SDK 自研实现替换为 rclone
fs 接口统一驱动。

- 新建 storage/rclone/ 包（~410 行胶水代码），包含通用 Provider 和 8 种
  配置映射 Factory
- 删除 10 个旧 provider 包（~1000 行），净减少约 1000 行代码
- StorageProvider 接口、前端 UI、数据库模型、备份执行引擎全部零改动
- 获得 rclone 工业级传输能力（分片上传、断点续传、自动重试）

server/internal/service/backup_execution_service_test.go

@@ -15,7 +15,7 @@ import (
 	"backupx/server/internal/repository"
 	"backupx/server/internal/storage"
 	"backupx/server/internal/storage/codec"
-	"backupx/server/internal/storage/localdisk"
+	storageRclone "backupx/server/internal/storage/rclone"
 )
 
 func newExecutionTestServices(t *testing.T) (*BackupExecutionService, *BackupRecordService, repository.BackupTaskRepository, repository.StorageTargetRepository, repository.BackupRecordRepository, string, string) {
@@ -53,7 +53,7 @@ func newExecutionTestServices(t *testing.T) (*BackupExecutionService, *BackupRec
 	}
 	logHub := backup.NewLogHub()
 	runnerRegistry := backup.NewRegistry(backup.NewFileRunner(), backup.NewMySQLRunner(nil), backup.NewSQLiteRunner(), backup.NewPostgreSQLRunner(nil))
-	storageRegistry := storage.NewRegistry(localdisk.NewFactory())
+	storageRegistry := storage.NewRegistry(storageRclone.NewLocalDiskFactory())
 	retentionService := backupretention.NewService(records)
 	tempDir := filepath.Join(baseDir, "tmp")
 	if err := os.MkdirAll(tempDir, 0o755); err != nil {