feat: initialize parent-bff service with full core features

add complete parent-bff implementation including:
- GraphQL endpoint with depth/cost validation
- ChildGuard越权校验 with redis cache and singleflight
- parallel orchestration with partial failure fallback
- three-level cache fallback strategy (Redis + LRU + downstream)
- Kafka consumer for cache invalidation and notification push
- opossum circuit breaker for downstream services
- Prometheus metrics and SLO alerts
- Helm chart for k8s deployment with multi-environment configs
- Grafana dashboard for observability
- complete unit and integration tests

services/parent-bff/src/app.module.ts

@@ -0,0 +1,18 @@
+import { Module } from "@nestjs/common";
+import { HealthModule } from "./shared/health/health.module.js";
+import { GraphqlModule } from "./graphql/graphql.module.js";
+import { ClientsModule } from "./clients/clients.module.js";
+import { KafkaModule } from "./shared/kafka/kafka.module.js";
+
+/**
+ * parent-bff 根模块。
+ *
+ * P4.1：HealthModule（/healthz + /readyz）
+ * P4.2：GraphqlModule（GraphQL Yoga /graphql 端点 + context middleware）
+ * P4.3：ClientsModule（下游 gRPC client 抽象层：iam + core-edu + data-ana + msg + push-http）
+ * P5：KafkaModule（事件订阅 + 缓存失效 + 通知推送）
+ */
+@Module({
+  imports: [HealthModule, GraphqlModule, ClientsModule, KafkaModule],
+})
+export class AppModule {}