feat: add comprehensive backend topics and fix build issues

## 新增内容

### 附录文档扩展
- 扩展前端项目架构文档 (frontend-project-architecture.md)
- 扩展后端项目架构文档 (backend-project-architecture.md)
- 扩展数据治理文档 (data-governance.md)
- 扩展数据可视化文档 (data-visualization.md)
- 扩展分布式系统文档 (distributed-systems.md)
- 扩展高可用文档 (high-availability.md)
- 扩展单体到微服务文档 (monolith-to-microservices.md)
- 扩展系统设计方法论文档 (system-design-methodology.md)
- 扩展 Docker 容器文档 (docker-containers.md)
- 扩展 Kubernetes 文档 (kubernetes.md)
- 扩展 Linux 基础文档 (linux-basics.md)
- 扩展神经网络文档 (neural-networks.md)

### 新增交互式组件
- 数据治理组件: DataQualityDemo, DataGovernanceFrameworkDemo, DataLineageDemo
- 数据可视化组件: ChartTypeSelectorDemo, DashboardLayoutDemo
- 分布式系统组件: CAPTheoremDemo, ConsistencyModelsDemo, DistributedChallengesDemo
- 高可用组件: AvailabilityCalculatorDemo, FailoverStrategyDemo
- 系统设计组件: SystemDesignStepsDemo, CapacityEstimationDemo
- Docker 容器组件: DockerArchitectureDemo, DockerLifecycleDemo
- Kubernetes 组件: K8sArchitectureDemo, K8sWorkloadsDemo
- Linux 基础组件: LinuxFileSystemDemo, LinuxCommandDemo, LinuxPermissionsDemo
- 神经网络组件: NeuronDemo, NetworkLayersDemo, NetworkArchitectureDemo
- 单体到微服务组件: ArchEvolutionDemo
- 项目架构组件: ProjectArchitectureComparisonDemo
- 附录导航组件: AppendixFlowMap

### 英文版重构
- 将 en-us 目录重命名为 en
- 更新相关配置和组件中的语言代码

## Bug 修复
- 修复 index.js 中重复的组件导入语句导致的 build 失败
- 恢复被注释的 InvertedIndexDemo 和 SearchRelevanceDemo 导入
- 修复 HomeFeatures.vue 中 en-us 与 config.mjs 中 en 不一致导致的语言切换问题

## 其他改进
- 添加构建脚本 (scripts/build.mjs)
- 更新依赖版本

docs/.vitepress/theme/components/appendix/high-availability/FailoverStrategyDemo.vue

@@ -0,0 +1,143 @@
+<!--
+  FailoverStrategyDemo.vue
+  故障转移策略演示：展示主备、主主、多活等高可用架构
+-->
+<template>
+  <div class="failover-demo">
+    <div class="header">
+      <div class="title">故障转移策略对比</div>
+      <div class="subtitle">点击查看不同高可用架构的工作方式</div>
+    </div>
+
+    <div class="strategy-tabs">
+      <div
+        v-for="s in strategies"
+        :key="s.key"
+        :class="['tab', { active: activeStrategy === s.key }]"
+        @click="activeStrategy = s.key"
+      >
+        {{ s.name }}
+      </div>
+    </div>
+
+    <div v-if="current" class="strategy-detail">
+      <div class="strategy-name">{{ current.name }}</div>
+      <div class="strategy-desc">{{ current.desc }}</div>
+
+      <div class="arch-visual">
+        <div
+          v-for="(node, i) in current.nodes"
+          :key="i"
+          :class="['arch-node', node.role]"
+        >
+          <div class="node-role">{{ node.label }}</div>
+          <div class="node-status">{{ node.status }}</div>
+        </div>
+      </div>
+
+      <div class="pros-cons">
+        <div class="pros">
+          <div class="pc-title">优点</div>
+          <div v-for="p in current.pros" :key="p" class="pc-item good">{{ p }}</div>
+        </div>
+        <div class="cons">
+          <div class="pc-title">缺点</div>
+          <div v-for="c in current.cons" :key="c" class="pc-item bad">{{ c }}</div>
+        </div>
+      </div>
+    </div>
+  </div>
+</template>
+
+<script setup>
+import { ref, computed } from 'vue'
+
+const activeStrategy = ref('active-standby')
+
+const strategies = [
+  {
+    key: 'active-standby',
+    name: '主备模式',
+    desc: '一个主节点处理所有请求，备节点待命。主节点故障时，备节点接管。',
+    nodes: [
+      { label: '主节点', status: '处理请求', role: 'primary' },
+      { label: '备节点', status: '待命同步', role: 'standby' }
+    ],
+    pros: ['架构简单，易于理解', '数据一致性好保证'],
+    cons: ['备节点资源浪费', '切换有短暂中断（秒级）']
+  },
+  {
+    key: 'active-active',
+    name: '主主模式',
+    desc: '两个节点都处理请求，互相同步数据。任一节点故障，另一个继续服务。',
+    nodes: [
+      { label: '节点 A', status: '处理请求', role: 'primary' },
+      { label: '节点 B', status: '处理请求', role: 'primary' }
+    ],
+    pros: ['资源利用率高', '无切换中断'],
+    cons: ['数据冲突处理复杂', '需要解决写冲突']
+  },
+  {
+    key: 'multi-az',
+    name: '多可用区',
+    desc: '在同一地域的不同数据中心部署，防止单个机房故障。',
+    nodes: [
+      { label: 'AZ-1 主', status: '读写', role: 'primary' },
+      { label: 'AZ-2 从', status: '只读', role: 'secondary' },
+      { label: 'AZ-3 从', status: '只读', role: 'secondary' }
+    ],
+    pros: ['机房级容灾', '读性能可扩展'],
+    cons: ['跨 AZ 延迟（1-2ms）', '成本增加']
+  },
+  {
+    key: 'multi-region',
+    name: '异地多活',
+    desc: '在不同地域部署完整的服务，每个地域独立处理本地流量。',
+    nodes: [
+      { label: '北京', status: '独立服务', role: 'primary' },
+      { label: '上海', status: '独立服务', role: 'primary' },
+      { label: '广州', status: '独立服务', role: 'primary' }
+    ],
+    pros: ['地域级容灾', '就近访问延迟低'],
+    cons: ['架构极其复杂', '数据同步挑战大']
+  }
+]
+
+const current = computed(() => strategies.find(s => s.key === activeStrategy.value))
+</script>
+
+<style scoped>
+.failover-demo {
+  border: 1px solid var(--vp-c-divider); background: var(--vp-c-bg-soft);
+  border-radius: 12px; padding: 1.5rem; margin: 1.5rem 0;
+}
+.header { margin-bottom: 1rem; }
+.title { font-weight: 700; font-size: 1.1rem; }
+.subtitle { color: var(--vp-c-text-2); font-size: 0.9rem; }
+.strategy-tabs { display: flex; gap: 0.5rem; margin-bottom: 1rem; flex-wrap: wrap; }
+.tab {
+  padding: 0.4rem 0.75rem; border-radius: 6px; cursor: pointer;
+  font-size: 0.85rem; background: var(--vp-c-bg); border: 1px solid var(--vp-c-divider);
+  transition: all 0.2s;
+}
+.tab:hover { border-color: var(--vp-c-brand); }
+.tab.active { border-color: var(--vp-c-brand); background: rgba(var(--vp-c-brand-rgb), 0.05); font-weight: 600; }
+.strategy-detail { background: var(--vp-c-bg); border-radius: 8px; padding: 1rem; border: 1px solid var(--vp-c-divider); }
+.strategy-name { font-weight: 700; font-size: 0.95rem; }
+.strategy-desc { color: var(--vp-c-text-2); font-size: 0.82rem; margin-bottom: 0.75rem; }
+.arch-visual { display: flex; gap: 0.5rem; margin-bottom: 0.75rem; flex-wrap: wrap; justify-content: center; }
+.arch-node {
+  padding: 0.5rem 0.75rem; border-radius: 6px; text-align: center;
+  border: 1px dashed var(--vp-c-divider); min-width: 90px;
+}
+.arch-node.primary { background: rgba(var(--vp-c-brand-rgb), 0.08); border-color: var(--vp-c-brand); }
+.arch-node.standby { background: rgba(245,158,11,0.08); border-color: #f59e0b; }
+.arch-node.secondary { background: rgba(99,102,241,0.08); border-color: #6366f1; }
+.node-role { font-weight: 700; font-size: 0.82rem; }
+.node-status { font-size: 0.72rem; color: var(--vp-c-text-2); }
+.pros-cons { display: grid; grid-template-columns: 1fr 1fr; gap: 0.5rem; }
+.pc-title { font-weight: 700; font-size: 0.82rem; margin-bottom: 0.3rem; }
+.pc-item { font-size: 0.78rem; padding: 0.2rem 0; }
+.pc-item.good::before { content: '+ '; color: #22c55e; font-weight: 700; }
+.pc-item.bad::before { content: '- '; color: #ef4444; font-weight: 700; }
+</style>