perf(worker): 缩短异步批量填槽延迟

将单次异步准入准备批次限制为 8 个任务，使双节点调度器以多个短事务批次渐进填满全局容量，避免准备 48 个媒体任务后才首次提交造成的长时间冷启动空槽。\n\n验证：Go 全量测试、go vet、runner/store race、gofmt、迁移安全检查通过。

apps/api/internal/runner/admission.go

@@ -33,6 +33,7 @@ const (
 	asyncWorkerCapacityScopeKey = "global"
 	asyncWorkerQueueLimit       = 10000
 	asyncWorkerMaxWaitSeconds   = 24 * 60 * 60
+	asyncAdmissionBatchMax      = 8
 	acceptanceQueueLimit        = 10000
 	acceptanceQueueMaxWait      = 15 * 60
 	synchronousAdmissionWakeMax = 256
@@ -707,8 +708,8 @@ func (s *Service) dispatchWaitingAsyncAdmissions(ctx context.Context) {
 		if batchLimit <= 0 {
 			batchLimit = 64
 		}
-		if batchLimit > 128 {
-			batchLimit = 128
+		if batchLimit > asyncAdmissionBatchMax {
+			batchLimit = asyncAdmissionBatchMax
 		}
 		taskIDs, err := s.store.ListWaitingAsyncAdmissionTaskIDs(ctx, batchLimit)
 		if err != nil {