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

infra/k8s/helm/parent-bff/Chart.yaml

@@ -0,0 +1,13 @@
+apiVersion: v2
+name: parent-bff
+description: parent-bff 服务级 Helm Chart（家长端 BFF 聚合层，GraphQL 端口 3010）
+type: application
+version: 0.1.0
+appVersion: "0.1.0"
+keywords:
+  - edu
+  - bff
+  - parent
+  - graphql
+maintainers:
+  - name: edu-arch