refactor: eliminate string/magic-number literals (goconst, mnd) and fix const-type grouping (SA9004)

- upload/task: taskCategoryUpload/taskQueueDefault 常量替代 8 处字面量
- admin: 复用既有 logDBNameSQLite 常量替代 3 处 "sqlite" 字面量
- pkg/cache/disk: defaultCleanupInterval 命名常量
- driver_asynq_worker/executor: 分离 contextKey 类型常量组
lint_issues 45→34, tests 44/44

backend/plugins/domain/upload/task/storage_migration.go

@@ -39,9 +39,9 @@ var StorageMigrationMeta = contracts.TaskMetaDTO{
 	Name:        StorageMigrationTask,
 	DisplayName: "迁移文件存储",
 	Description: "将活动存储中的文件迁移到待切换的目标存储，迁移期间文件系统保持只读",
-	Category:    "upload",
+	Category:    taskCategoryUpload,
 	MaxRetry:    3,
-	Queue:       "default",
+	Queue:       taskQueueDefault,
 	Params: []contracts.TaskParamDTO{
 		{
 			Name:        "target",