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/retry_decision.go

@@ -160,6 +160,15 @@ func resolveCandidateFailure(input resolveCandidateFailureInput) failureDecision
 		}
 	}
 	if errors.Is(input.Err, store.ErrRateLimited) {
+		if _, platformLimited := platformModelRateLimitError(input.Err); platformLimited && input.HasNextCandidate {
+			return failureDecision{
+				Route:  "next",
+				Effect: "none",
+				Reason: "quota_race_rotated",
+				Match:  policyRuleMatch{Source: "gateway_rate_limits", Policy: "rateLimitPolicy", Rule: "platformModelCapacity", Value: "saturated"},
+				Info:   info,
+			}
+		}
 		route := "stop"
 		if input.Async && store.RateLimitRetryable(input.Err) {
 			route = "requeue"