feat(routing): 完善平台满载避让与故障轮转

为未配置并发上限的平台增加基于运行和等待任务数的软负载，并按非满载、有效优先级、缓存亲和与负载稳定排序。\n\n平台模型 RPM、TPM 和并发额度竞争失败时只轮转候选，不触发冷却、禁用或降级；用户组额度保持不可绕过。补齐异步冷却排队恢复、满载原因、选择原因与低基数指标。\n\n验证：go test ./...；go vet ./...；PostgreSQL 原子额度、准入队列、Worker 容量回收及故障策略 HTTP acceptance。

apps/api/internal/runner/admission.go

@@ -123,11 +123,12 @@ func (s *Service) buildTaskAdmissionPlanForCurrentBinding(
 	candidates, _ = pinCandidatesToTaskAdmission(candidates, admission)
 	var candidateRateLimitErr error
 	for _, candidate := range candidates {
-		available, availabilityErr := s.store.RuntimeCandidateAvailable(ctx, candidate.PlatformID, candidate.PlatformModelID)
+		available, unavailableReason, availabilityErr := s.store.RuntimeCandidateAvailability(ctx, candidate.PlatformID, candidate.PlatformModelID)
 		if availabilityErr != nil {
 			return taskAdmissionPlan{}, availabilityErr
 		}
 		if !available {
+			s.observeCandidateRouting(unavailableReason)
 			continue
 		}
 		scopes, groupID, scopeErr := s.taskAdmissionScopes(ctx, task, user, candidate)
@@ -141,13 +142,6 @@ func (s *Service) buildTaskAdmissionPlanForCurrentBinding(
 				break
 			}
 		}
-		if !hasConcurrentLimit {
-			return taskAdmissionPlan{
-				Candidate: candidate,
-				Body:      body,
-				ModelType: modelType,
-			}, nil
-		}
 		reservations := acceptanceInfrastructureReservations(
 			task,
 			s.rateLimitReservations(ctx, user, candidate, body),
@@ -162,6 +156,13 @@ func (s *Service) buildTaskAdmissionPlanForCurrentBinding(
 			}
 			return taskAdmissionPlan{}, err
 		}
+		if !hasConcurrentLimit {
+			return taskAdmissionPlan{
+				Candidate: candidate,
+				Body:      body,
+				ModelType: modelType,
+			}, nil
+		}
 		return taskAdmissionPlan{
 			Candidate: candidate,
 			Body:      body,
@@ -172,6 +173,7 @@ func (s *Service) buildTaskAdmissionPlanForCurrentBinding(
 		}, nil
 	}
 	if candidateRateLimitErr != nil {
+		s.observeCandidateRouting("all_full_queued")
 		return taskAdmissionPlan{}, candidateRateLimitErr
 	}
 	return taskAdmissionPlan{}, store.ErrNoModelCandidate
@@ -475,12 +477,6 @@ func (s *Service) ensureCandidateAdmission(
 			break
 		}
 	}
-	if !hasConcurrentLimit {
-		if err := s.store.DeleteTaskAdmission(context.WithoutCancel(ctx), task.ID); err != nil && !errors.Is(err, pgx.ErrNoRows) {
-			return store.TaskAdmissionResult{}, false, err
-		}
-		return store.TaskAdmissionResult{}, false, nil
-	}
 	reservations := acceptanceInfrastructureReservations(
 		task,
 		s.rateLimitReservations(ctx, user, candidate, body),
@@ -488,6 +484,12 @@ func (s *Service) ensureCandidateAdmission(
 	if err := s.store.CheckRateLimits(ctx, reservations); err != nil {
 		return store.TaskAdmissionResult{}, true, err
 	}
+	if !hasConcurrentLimit {
+		if err := s.store.DeleteTaskAdmission(context.WithoutCancel(ctx), task.ID); err != nil && !errors.Is(err, pgx.ErrNoRows) {
+			return store.TaskAdmissionResult{}, false, err
+		}
+		return store.TaskAdmissionResult{}, false, nil
+	}
 	plan := taskAdmissionPlan{
 		Candidate: candidate,
 		Body:      body,
@@ -809,6 +811,7 @@ func (s *Service) dispatchWaitingAsyncTasks(ctx context.Context, admissions []st
 				return false, outcome.Err
 			}
 			if !outcome.Result.Admitted {
+				s.observeCandidateRouting("quota_race_rotated")
 				platformModelID := outcome.Result.Admission.PlatformModelID
 				if platformModelID == "" {
 					for _, input := range inputs {