feat(cluster): 增加跨节点任务执行与请求素材场景

新增 AI_GATEWAY_ASYNC_QUEUE_WORKER_ENABLED，默认保持启用；关闭时仅停止 River 异步任务执行器，HTTP、健康检查、结算与其他后台任务继续运行。

新增 0089 迁移并同步管理端默认场景，将 request_asset 加入现有上传渠道，确保 multipart 请求素材先转换为跨节点共享 URL。

验证：gofmt、go test ./... -count=1、go vet ./...、pnpm lint、pnpm test、pnpm build、迁移测试。

apps/api/internal/config/config.go

@@ -57,6 +57,7 @@ type Config struct {
 	GlobalHTTPProxySource                         string
 	LogLevel                                      slog.Level
 	BillingEngineMode                             string
+	AsyncQueueWorkerEnabled                       bool
 	AsyncWorkerHardLimit                          int
 	AsyncWorkerRefreshIntervalSeconds             int
 }
@@ -112,6 +113,7 @@ func Load() Config {
 		GlobalHTTPProxySource:             globalProxy.Source,
 		LogLevel:                          logLevel(env("LOG_LEVEL", "info")),
 		BillingEngineMode:                 strings.ToLower(env("BILLING_ENGINE_MODE", "observe")),
+		AsyncQueueWorkerEnabled:           env("AI_GATEWAY_ASYNC_QUEUE_WORKER_ENABLED", "true") == "true",
 		AsyncWorkerHardLimit:              envIntValidated("AI_GATEWAY_ASYNC_WORKER_HARD_LIMIT", 2048),
 		AsyncWorkerRefreshIntervalSeconds: envIntValidated("AI_GATEWAY_ASYNC_WORKER_REFRESH_INTERVAL_SECONDS", 5),
 	}