feat(p6): production hardening with circuit breaker, backup, monitoring and chaos engineering

P6 生产硬化阶段交付物（46 文件）：

## 1. API Gateway 中间件链（services/api-gateway/internal/middleware/）
- circuit-breaker.go: gobreaker v2 熔断器（5s 窗口/50% 错误率/30s OPEN→HALF_OPEN）
- ratelimit.go: 令牌桶限流（sync.Map + cleanup goroutine，默认 100rps/20 burst）
- cors.go: CORS 中间件（CORS_ORIGINS 环境变量）
- recovery.go: panic 恢复 + uuid request_id
- security.go: 安全头 + 请求体 10MB 限制
- requestid.go: 请求 ID 注入
- health/health.go: /healthz + /readyz 健康检查
- main.go: 重写注册全部中间件链（Recovery→RequestID→CORS→Security→BodyLimit→RateLimit→CircuitBreaker→Auth）

## 2. 基础设施硬化（infra/）
- backup/backup-mysql.sh: MySQL 全量备份（mysqldump+gzip，按服务独立）
- backup/restore-mysql.sh: 恢复脚本
- backup/backup-cron.sh: cron 调度入口（5 服务批量备份）
- alertmanager/alertmanager.yml: 告警路由（webhook + 邮件示例）
- prometheus/rules.yml: 8 条告警规则（服务可用性/性能/资源 3 组）
- grafana/dashboards/microservices-overview.json: 4 panel 仪表盘
- grafana/provisioning/: 数据源和仪表盘 provisioning
- k8s/namespace.yaml: 4 命名空间（edu-system/services/monitoring/ingress）
- k8s/api-gateway-deployment.yaml: Deployment + Service 骨架
- chaos/experiments.yaml: 3 个 Litmus 混沌实验（pod-kill/network-latency/disk-fill）
- docker-compose.monitoring.yml: 监控栈 profile
- security/secrets.example.env: 8 项密钥占位符
- security/waf-rules.conf: ModSecurity WAF 规则骨架

## 3. 业务服务健康检查 + 优雅停机（5 个 NestJS 服务）
- services/{iam,core-edu,content,msg,classes}/src/shared/health/: /healthz + /readyz
- services/{iam,core-edu,content,msg,classes}/src/shared/lifecycle/: OnModuleInit + OnApplicationShutdown

## 4. Python 服务健康检查
- services/{ai,data-ana}/src/health/health.py: FastAPI APIRouter

## 5. 运维文档
- docs/architecture/runbooks/p6-hardening.md: P6 总览 Runbook（9 章节）
- docs/architecture/runbooks/incident-response.md: 事件响应手册（5 章节）
- docs/architecture/004-p6-addendum.md: 004 架构补记 P6 章节
- docs/troubleshooting/known-issues-p6-addendum.md: 15 条 P6 场景→技术映射

## 验收信号
- RPO ≤ 15min（MySQL 备份 + binlog PITR）
- RTO ≤ 30min（K8s 滚动更新 + DNS 切换）
- P99 ≤ 500ms（熔断 + 限流 + 缓存）
- 熔断器错误率 > 50% 触发 OPEN
- 限流 100rps/20 burst
- 备份保留 7 天
- 混沌实验每月 1 次

infra/k8s/api-gateway-deployment.yaml

@@ -0,0 +1,125 @@
+# api-gateway Deployment + Service 骨架
+# 生产环境请通过 Helm Chart 管理，此处仅作骨架参考
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: api-gateway
+  namespace: edu-services
+  labels:
+    app.kubernetes.io/name: api-gateway
+    app.kubernetes.io/part-of: edu-platform
+    app.kubernetes.io/component: gateway
+spec:
+  replicas: 2
+  selector:
+    matchLabels:
+      app.kubernetes.io/name: api-gateway
+  strategy:
+    type: RollingUpdate
+    rollingUpdate:
+      maxSurge: 1
+      maxUnavailable: 0
+  template:
+    metadata:
+      labels:
+        app.kubernetes.io/name: api-gateway
+        app.kubernetes.io/part-of: edu-platform
+        app.kubernetes.io/component: gateway
+      annotations:
+        prometheus.io/scrape: "true"
+        prometheus.io/port: "8080"
+        prometheus.io/path: "/metrics"
+    spec:
+      containers:
+        - name: api-gateway
+          image: edu/api-gateway:latest  # 生产请固定 tag，避免 latest
+          imagePullPolicy: IfNotPresent
+          ports:
+            - name: http
+              containerPort: 8080
+              protocol: TCP
+          # 存活探针：失败触发重启
+          livenessProbe:
+            httpGet:
+              path: /healthz
+              port: http
+            initialDelaySeconds: 15
+            periodSeconds: 20
+            timeoutSeconds: 3
+            failureThreshold: 3
+          # 就绪探针：失败从 Service Endpoints 摘除
+          readinessProbe:
+            httpGet:
+              path: /readyz
+              port: http
+            initialDelaySeconds: 5
+            periodSeconds: 10
+            timeoutSeconds: 3
+            failureThreshold: 2
+          resources:
+            requests:
+              cpu: "250m"
+              memory: "256Mi"
+            limits:
+              cpu: "1000m"
+              memory: "1Gi"
+          env:
+            # 从 ConfigMap 引用非敏感配置
+            - name: NODE_ENV
+              valueFrom:
+                configMapKeyRef:
+                  name: api-gateway-config
+                  key: NODE_ENV
+            - name: LOG_LEVEL
+              valueFrom:
+                configMapKeyRef:
+                  name: api-gateway-config
+                  key: LOG_LEVEL
+            - name: MYSQL_HOST
+              valueFrom:
+                configMapKeyRef:
+                  name: api-gateway-config
+                  key: MYSQL_HOST
+            # 从 Secret 引用敏感配置
+            - name: MYSQL_PASSWORD
+              valueFrom:
+                secretKeyRef:
+                  name: api-gateway-secret
+                  key: MYSQL_PASSWORD
+            - name: JWT_SECRET
+              valueFrom:
+                secretKeyRef:
+                  name: api-gateway-secret
+                  key: JWT_SECRET
+            - name: REDIS_PASSWORD
+              valueFrom:
+                secretKeyRef:
+                  name: api-gateway-secret
+                  key: REDIS_PASSWORD
+          # 生产建议挂载 /tmp 并设置 readOnlyRootFilesystem: true
+          securityContext:
+            runAsNonRoot: true
+            runAsUser: 1000
+            allowPrivilegeEscalation: false
+            capabilities:
+              drop: ["ALL"]
+---
+# api-gateway Service（ClusterIP）
+apiVersion: v1
+kind: Service
+metadata:
+  name: api-gateway
+  namespace: edu-services
+  labels:
+    app.kubernetes.io/name: api-gateway
+    app.kubernetes.io/part-of: edu-platform
+    app.kubernetes.io/component: gateway
+spec:
+  type: ClusterIP
+  selector:
+    app.kubernetes.io/name: api-gateway
+  ports:
+    - name: http
+      port: 8080
+      targetPort: http
+      protocol: TCP