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/edu-platform/templates/ingress.yaml

@@ -0,0 +1,35 @@
+{{- if .Values.ingress.enabled }}
+{{- range .Values.ingress.hosts }}
+---
+apiVersion: networking.k8s.io/v1
+kind: Ingress
+metadata:
+  name: {{ $.Release.Name }}-ingress
+  namespace: edu-ingress
+  labels:
+    {{- include "edu-platform.labels" $ | nindent 4 }}
+  {{- with $.Values.ingress.annotations }}
+  annotations:
+    {{- toYaml . | nindent 4 }}
+  {{- end }}
+spec:
+  ingressClassName: {{ $.Values.ingress.className }}
+  {{- with $.Values.ingress.tls }}
+  tls:
+    {{- toYaml . | nindent 4 }}
+  {{- end }}
+  rules:
+    - host: {{ .host }}
+      http:
+        paths:
+          {{- range .paths }}
+          - path: {{ .path }}
+            pathType: {{ .pathType }}
+            backend:
+              service:
+                name: {{ .service }}
+                port:
+                  number: {{ .port }}
+          {{- end }}
+{{- end }}
+{{- end }}