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/templates/_helpers.tpl

@@ -0,0 +1,29 @@
+{{- define "parent-bff.name" -}}
+{{- default .Chart.Name .Values.nameOverride | trunc 63 | trimSuffix "-" -}}
+{{- end -}}
+
+{{- define "parent-bff.fullname" -}}
+{{- if .Values.fullnameOverride -}}
+{{- .Values.fullnameOverride | trunc 63 | trimSuffix "-" -}}
+{{- else -}}
+{{- $name := default .Chart.Name .Values.nameOverride -}}
+{{- if contains $name .Release.Name -}}
+{{- .Release.Name | trunc 63 | trimSuffix "-" -}}
+{{- else -}}
+{{- printf "%s-%s" .Release.Name $name | trunc 63 | trimSuffix "-" -}}
+{{- end -}}
+{{- end -}}
+{{- end -}}
+
+{{- define "parent-bff.labels" -}}
+helm.sh/chart: {{ .Chart.Name }}-{{ .Chart.Version | replace "+" "_" }}
+{{ include "parent-bff.selectorLabels" . }}
+app.kubernetes.io/managed-by: {{ .Release.Service }}
+app.kubernetes.io/part-of: edu-platform
+app.kubernetes.io/component: bff
+{{- end -}}
+
+{{- define "parent-bff.selectorLabels" -}}
+app.kubernetes.io/name: {{ include "parent-bff.name" . }}
+app.kubernetes.io/instance: {{ .Release.Name }}
+{{- end -}}