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/store/runtime_policies.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"database/sql"
 	"encoding/json"
+	"errors"
 	"strings"
 	"time"
 
@@ -200,25 +201,30 @@ WHERE id = $1::uuid`, platformModelID, cooldownSeconds)
 	return err
 }
 
-func (s *Store) RuntimeCandidateAvailable(ctx context.Context, platformID string, platformModelID string) (bool, error) {
+func (s *Store) RuntimeCandidateAvailability(ctx context.Context, platformID string, platformModelID string) (bool, string, error) {
 	if strings.TrimSpace(platformID) == "" || strings.TrimSpace(platformModelID) == "" {
-		return false, nil
+		return false, "disabled_skipped", nil
 	}
-	var available bool
+	var reason string
 	err := s.pool.QueryRow(ctx, `
-SELECT EXISTS (
-  SELECT 1
-  FROM platform_models model
-  JOIN integration_platforms platform ON platform.id = model.platform_id
-  WHERE platform.id = $1::uuid
-    AND model.id = $2::uuid
-    AND platform.status = 'enabled'
-    AND platform.deleted_at IS NULL
-    AND (platform.cooldown_until IS NULL OR platform.cooldown_until <= now())
-    AND model.enabled = true
-    AND (model.cooldown_until IS NULL OR model.cooldown_until <= now())
-)`, platformID, platformModelID).Scan(&available)
-	return available, err
+SELECT CASE
+  WHEN platform.deleted_at IS NOT NULL OR platform.status <> 'enabled' OR model.enabled = false
+    THEN 'disabled_skipped'
+  WHEN platform.cooldown_until > now() OR model.cooldown_until > now()
+    THEN 'cooldown_skipped'
+  ELSE ''
+END
+FROM platform_models model
+JOIN integration_platforms platform ON platform.id = model.platform_id
+WHERE platform.id = $1::uuid
+  AND model.id = $2::uuid`, platformID, platformModelID).Scan(&reason)
+	if errors.Is(err, pgx.ErrNoRows) {
+		return false, "disabled_skipped", nil
+	}
+	if err != nil {
+		return false, "", err
+	}
+	return reason == "", reason, nil
 }
 
 func (s *Store) ApplyCandidateFailureEffect(ctx context.Context, input CandidateFailureEffectInput) (CandidateFailureEffectResult, error) {