fix(routing): 避免路由探测耗尽数据库连接

将跨实例探测协调从持有会话级 advisory lock 改为带过期时间的数据库短租约，避免网络探测期间占用关键 PostgreSQL 连接。新增可配置的探测并发上限和真实 PostgreSQL 回归测试，确保租约生效期间连接池仍可全部获取。\n\n验证：go test ./... -count=1；go vet ./...；真实 PostgreSQL 17 集成测试；迁移安全检查。

apps/api/internal/config/config.go

@@ -101,6 +101,7 @@ type Config struct {
 	RouteProbeHotIntervalSeconds                  int
 	RouteProbeColdIntervalSeconds                 int
 	RouteProbeTimeoutMS                           int
+	RouteProbeConcurrency                         int
 	WorkerAutoscalingEnabled                      bool
 	CapacityOrchestratorAdapter                   string
 	CapacityPoolsJSON                             string
@@ -223,6 +224,7 @@ func Load() Config {
 		RouteProbeHotIntervalSeconds:      envIntValidated("AI_GATEWAY_ROUTE_PROBE_HOT_INTERVAL_SECONDS", 15),
 		RouteProbeColdIntervalSeconds:     envIntValidated("AI_GATEWAY_ROUTE_PROBE_COLD_INTERVAL_SECONDS", 60),
 		RouteProbeTimeoutMS:               envIntValidated("AI_GATEWAY_ROUTE_PROBE_TIMEOUT_MS", 3000),
+		RouteProbeConcurrency:             envIntValidated("AI_GATEWAY_ROUTE_PROBE_CONCURRENCY", 8),
 		WorkerAutoscalingEnabled:          env("AI_GATEWAY_WORKER_AUTOSCALING_ENABLED", "false") == "true",
 		CapacityOrchestratorAdapter:       strings.ToLower(strings.TrimSpace(env("AI_GATEWAY_CAPACITY_ORCHESTRATOR_ADAPTER", "kubernetes"))),
 		CapacityPoolsJSON:                 strings.TrimSpace(env("AI_GATEWAY_CAPACITY_POOLS", "")),
@@ -345,6 +347,9 @@ func (c Config) Validate() error {
 	if c.RouteProbeTimeoutMS != 0 && (c.RouteProbeTimeoutMS < 250 || c.RouteProbeTimeoutMS > 10000) {
 		return errors.New("AI_GATEWAY_ROUTE_PROBE_TIMEOUT_MS must be between 250 and 10000")
 	}
+	if c.RouteProbeConcurrency != 0 && (c.RouteProbeConcurrency < 1 || c.RouteProbeConcurrency > 64) {
+		return errors.New("AI_GATEWAY_ROUTE_PROBE_CONCURRENCY must be between 1 and 64")
+	}
 	if strings.EqualFold(c.RoutingMode, "enforced") && len(c.WorkerExecutionSecret) < 32 {
 		return errors.New("AI_GATEWAY_WORKER_EXECUTION_SECRET must be at least 32 bytes in enforced routing mode")
 	}