fix(worker): 容量饱和后停止重复准入扫描

双 Worker 每秒遍历全部 waiting 任务，在全局执行槽已满时仍重复探测，导致 River 执行协程长时间等待同一 worker_capacity 进程锁。

异步 FIFO 头部未准入时立即结束本轮扫描，等待租约释放通知后再继续；任务级错误仍跳过并处理后续任务。

验证：Go 全量测试、runner race、go vet、迁移安全检查通过。

apps/api/internal/runner/admission.go

@@ -603,19 +603,25 @@ func (s *Service) SubmitAsyncTask(ctx context.Context, task store.GatewayTask) e
 	return nil
 }
 
-func (s *Service) dispatchWaitingAsyncTask(ctx context.Context, task store.GatewayTask) error {
+func (s *Service) dispatchWaitingAsyncTask(ctx context.Context, task store.GatewayTask) (bool, error) {
 	user := authUserFromTask(task)
 	plan, err := s.buildTaskAdmissionPlan(ctx, task, user)
 	if err != nil {
-		return err
+		return false, err
 	}
 	if !plan.Eligible {
-		return s.EnqueueAsyncTask(ctx, task)
+		if err := s.EnqueueAsyncTask(ctx, task); err != nil {
+			return false, err
+		}
+		return true, nil
 	}
-	_, err = s.tryTaskAdmissionWithAdmittedHook(ctx, task, plan, "", func(tx pgx.Tx) error {
+	result, err := s.tryTaskAdmissionWithAdmittedHook(ctx, task, plan, "", func(tx pgx.Tx) error {
 		return s.enqueueAsyncTaskTx(ctx, tx, task.ID, asyncTaskInsertOpts(task))
 	})
-	return err
+	if err != nil {
+		return false, err
+	}
+	return result.Admitted, nil
 }
 
 func (s *Service) cancelAsyncSubmissionIfDisconnected(ctx context.Context, taskID string) bool {
@@ -660,8 +666,17 @@ func (s *Service) dispatchWaitingAsyncAdmissions(ctx context.Context) {
 			if task.Status != "queued" {
 				continue
 			}
-			if err := s.dispatchWaitingAsyncTask(ctx, task); err != nil {
+			admitted, err := s.dispatchWaitingAsyncTask(ctx, task)
+			if err != nil {
 				s.logger.Warn("dispatch waiting async admission failed", "taskID", taskID, "error", err)
+				continue
+			}
+			if !admitted {
+				// Every asynchronous task shares the global worker-capacity FIFO
+				// scope. Once its current head cannot be admitted, later tasks
+				// cannot make progress until a lease is released.
+				s.observeTaskAdmission("dispatch_saturated")
+				break
 			}
 		}
 	}