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/pkg/cache/disk/cache.go

@@ -91,13 +91,16 @@ func New(basePath string) *Cache {
 var (
 	defaultCache     *Cache
 	defaultCacheOnce sync.Once
+
+	// defaultCleanupInterval 全局磁盘缓存默认的过期清理巡检周期
+	defaultCleanupInterval = 10 * time.Minute
 )
 
 // Default returns the default global disk cache instance.
 func Default() *Cache {
 	defaultCacheOnce.Do(func() {
 		defaultCache = New("uploads/diskcache")
-		go defaultCache.StartCleanupWorker(10 * time.Minute)
+		go defaultCache.StartCleanupWorker(defaultCleanupInterval)
 	})
 	return defaultCache
 }