feat(infra): k8s Helm Chart 演化与备份脚本测试

- 新增 edu-platform 平台级 chart（namespace/configmap/secret/ingress/hpa）
- 新增 api-gateway 服务级 chart（完整迁移自原 manifest）
- 新增 6 个业务服务 chart 桩（iam/core-edu/content/msg/data-ana/ai）
- 删除原 api-gateway-deployment.yaml（已迁移至 helm chart）
- 更新 infra/k8s/README.md 为 Helm Chart 管理说明
- 新增 backup-mysql.sh dry-run 测试脚本（17 断言）

infra/k8s/helm/api-gateway/templates/hpa.yaml

@@ -0,0 +1,29 @@
+{{- if .Values.hpa.enabled }}
+apiVersion: autoscaling/v2
+kind: HorizontalPodAutoscaler
+metadata:
+  name: {{ include "api-gateway.name" . }}-hpa
+  namespace: {{ .Values.namespace }}
+  labels:
+    {{- include "api-gateway.labels" . | nindent 4 }}
+spec:
+  scaleTargetRef:
+    apiVersion: apps/v1
+    kind: Deployment
+    name: {{ include "api-gateway.name" . }}
+  minReplicas: {{ .Values.hpa.minReplicas }}
+  maxReplicas: {{ .Values.hpa.maxReplicas }}
+  metrics:
+    - type: Resource
+      resource:
+        name: cpu
+        target:
+          type: Utilization
+          averageUtilization: {{ .Values.hpa.targetCPUUtilizationPercentage }}
+    - type: Resource
+      resource:
+        name: memory
+        target:
+          type: Utilization
+          averageUtilization: {{ .Values.hpa.targetMemoryUtilizationPercentage }}
+{{- end }}