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/securityevents/metrics_test.go

@@ -46,6 +46,12 @@ func TestMetricsExposeBoundedOutcomesAndState(t *testing.T) {
 	metrics.SetWorkerLoad(12, 3, 8, 2, 5, 1, "busy")
 	metrics.ObserveProviderQuotaWait("rpm")
 	metrics.ObserveProviderQuotaWait("tpm_total")
+	metrics.ObserveCandidateRouting("normal_rotation")
+	metrics.ObserveCandidateRouting("full_avoided")
+	metrics.ObserveCandidateRouting("quota_race_rotated")
+	metrics.ObserveCandidateRouting("cooldown_skipped")
+	metrics.ObserveCandidateRouting("disabled_skipped")
+	metrics.ObserveCandidateRouting("all_full_queued")
 	metrics.ObserveAsyncWorkerResize("success")
 	metrics.ObserveConcurrencyLeaseRenewal("success")
 	metrics.ObserveConcurrencyLeaseRenewal("lost")
@@ -86,6 +92,12 @@ func TestMetricsExposeBoundedOutcomesAndState(t *testing.T) {
 		`easyai_gateway_worker_pressure_state 1`,
 		`easyai_gateway_provider_quota_waits_total{outcome="rpm"} 1`,
 		`easyai_gateway_provider_quota_waits_total{outcome="tpm"} 1`,
+		`easyai_gateway_candidate_routing_total{outcome="normal_rotation"} 1`,
+		`easyai_gateway_candidate_routing_total{outcome="full_avoided"} 1`,
+		`easyai_gateway_candidate_routing_total{outcome="quota_race_rotated"} 1`,
+		`easyai_gateway_candidate_routing_total{outcome="cooldown_skipped"} 1`,
+		`easyai_gateway_candidate_routing_total{outcome="disabled_skipped"} 1`,
+		`easyai_gateway_candidate_routing_total{outcome="all_full_queued"} 1`,
 		`easyai_gateway_platform_model_rate_limit_utilization{platform_model_id="platform-model-1",metric="concurrent"} 0.750000`,
 		`easyai_gateway_async_worker_resizes_total{outcome="success"} 1`,
 		`easyai_gateway_concurrency_lease_renewals_total{outcome="success"} 1`,