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_db.go

@@ -146,7 +146,7 @@ func getSQLiteOverview(gormDB *gorm.DB) (DBOverviewResponse, error) {
 	}
 
 	return DBOverviewResponse{
-		Type:        "sqlite",
+		Type:        logDBNameSQLite,
 		Version:     version,
 		Name:        name,
 		Size:        sizeStr,
@@ -499,7 +499,7 @@ func ExecuteSQL(c *gin.Context) {
 
 func getSQLiteInfo(ctx context.Context) DatabaseInfoResponse {
 	info := DatabaseInfoResponse{
-		Type:    "sqlite",
+		Type:    logDBNameSQLite,
 		Name:    config.Config.Database.SQLitePath,
 		Version: "SQLite",
 	}