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/admin/handlers_status.go

@@ -175,7 +175,7 @@ type LogDatabaseStatus struct {
 // @Router /api/v1/admin/status/log-database [get]
 func GetLogDatabaseStatus(c *gin.Context) {
 	ctx := c.Request.Context()
-	activeDB := "sqlite"
+	activeDB := logDBNameSQLite
 	migration := "idle"
 	if rc := GetRiskControlService(); rc != nil {
 		activeDB = rc.ActiveLogEngine(ctx)