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/system-design-methodology/CapacityEstimationDemo.vue

@@ -0,0 +1,183 @@
+<!--
+  CapacityEstimationDemo.vue
+  容量估算计算器：信封背面估算交互演示
+-->
+<template>
+  <div class="capacity-demo">
+    <div class="header">
+      <div class="title">信封背面估算器</div>
+      <div class="subtitle">输入基础数据，自动计算系统容量需求</div>
+    </div>
+
+    <div class="inputs">
+      <div class="input-group">
+        <label>日活用户（万）</label>
+        <input v-model.number="dau" type="number" min="1" max="100000" />
+      </div>
+      <div class="input-group">
+        <label>人均请求数/天</label>
+        <input v-model.number="reqPerUser" type="number" min="1" max="1000" />
+      </div>
+      <div class="input-group">
+        <label>单次响应大小（KB）</label>
+        <input v-model.number="responseSize" type="number" min="0.1" max="1000" />
+      </div>
+      <div class="input-group">
+        <label>峰值系数</label>
+        <input v-model.number="peakFactor" type="number" min="1" max="10" step="0.5" />
+      </div>
+    </div>
+
+    <div class="results">
+      <div class="result-card">
+        <div class="result-label">日请求量</div>
+        <div class="result-value">{{ formatNumber(dailyRequests) }}</div>
+      </div>
+      <div class="result-card">
+        <div class="result-label">平均 QPS</div>
+        <div class="result-value">{{ formatNumber(avgQps) }}</div>
+      </div>
+      <div class="result-card">
+        <div class="result-label">峰值 QPS</div>
+        <div class="result-value">{{ formatNumber(peakQps) }}</div>
+      </div>
+      <div class="result-card">
+        <div class="result-label">日带宽</div>
+        <div class="result-value">{{ formatBandwidth(dailyBandwidth) }}</div>
+      </div>
+      <div class="result-card">
+        <div class="result-label">峰值带宽</div>
+        <div class="result-value">{{ formatBandwidth(peakBandwidthPerSec) }}/s</div>
+      </div>
+    </div>
+
+    <div class="reference">
+      <div class="ref-title">常用估算参考值</div>
+      <div class="ref-grid">
+        <div class="ref-item" v-for="r in references" :key="r.label">
+          <span class="ref-label">{{ r.label }}</span>
+          <span class="ref-value">{{ r.value }}</span>
+        </div>
+      </div>
+    </div>
+  </div>
+</template>
+
+<script setup>
+import { ref, computed } from 'vue'
+
+const dau = ref(100)
+const reqPerUser = ref(20)
+const responseSize = ref(5)
+const peakFactor = ref(3)
+
+const dailyRequests = computed(() => dau.value * 10000 * reqPerUser.value)
+const avgQps = computed(() => Math.round(dailyRequests.value / 86400))
+const peakQps = computed(() => Math.round(avgQps.value * peakFactor.value))
+const dailyBandwidth = computed(() => dailyRequests.value * responseSize.value * 1024)
+const peakBandwidthPerSec = computed(() => peakQps.value * responseSize.value * 1024)
+
+const references = [
+  { label: '1 天', value: '86,400 秒' },
+  { label: '1 月', value: '≈ 250 万秒' },
+  { label: 'QPS 1000', value: '≈ 1 台 8 核服务器' },
+  { label: '1 亿/天', value: '≈ 1,200 QPS' },
+  { label: 'MySQL 单机', value: '≈ 5,000 QPS' },
+  { label: 'Redis 单机', value: '≈ 100,000 QPS' }
+]
+
+function formatNumber(n) {
+  if (n >= 1e8) return (n / 1e8).toFixed(1) + ' 亿'
+  if (n >= 1e4) return (n / 1e4).toFixed(1) + ' 万'
+  return n.toLocaleString()
+}
+
+function formatBandwidth(bytes) {
+  if (bytes >= 1e12) return (bytes / 1e12).toFixed(1) + ' TB'
+  if (bytes >= 1e9) return (bytes / 1e9).toFixed(1) + ' GB'
+  if (bytes >= 1e6) return (bytes / 1e6).toFixed(1) + ' MB'
+  if (bytes >= 1e3) return (bytes / 1e3).toFixed(1) + ' KB'
+  return bytes + ' B'
+}
+</script>
+
+<style scoped>
+.capacity-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; }
+.inputs {
+  display: grid;
+  grid-template-columns: repeat(auto-fit, minmax(140px, 1fr));
+  gap: 0.75rem;
+  margin-bottom: 1rem;
+}
+.input-group label {
+  display: block;
+  font-size: 0.78rem;
+  font-weight: 600;
+  color: var(--vp-c-text-2);
+  margin-bottom: 0.25rem;
+}
+.input-group input {
+  width: 100%;
+  padding: 0.4rem 0.5rem;
+  border: 1px solid var(--vp-c-divider);
+  border-radius: 6px;
+  font-size: 0.85rem;
+  background: var(--vp-c-bg);
+  color: var(--vp-c-text-1);
+}
+.results {
+  display: grid;
+  grid-template-columns: repeat(auto-fit, minmax(120px, 1fr));
+  gap: 0.5rem;
+  margin-bottom: 1rem;
+}
+.result-card {
+  padding: 0.6rem;
+  border-radius: 8px;
+  text-align: center;
+  background: var(--vp-c-bg);
+  border: 1px solid var(--vp-c-divider);
+}
+.result-label {
+  font-size: 0.72rem;
+  color: var(--vp-c-text-3);
+}
+.result-value {
+  font-size: 0.95rem;
+  font-weight: 700;
+  color: var(--vp-c-brand);
+}
+.reference {
+  background: var(--vp-c-bg);
+  border-radius: 8px;
+  padding: 0.75rem;
+  border: 1px solid var(--vp-c-divider);
+}
+.ref-title {
+  font-weight: 600;
+  font-size: 0.82rem;
+  margin-bottom: 0.5rem;
+}
+.ref-grid {
+  display: grid;
+  grid-template-columns: repeat(auto-fit, minmax(140px, 1fr));
+  gap: 0.3rem;
+}
+.ref-item {
+  font-size: 0.75rem;
+  display: flex;
+  justify-content: space-between;
+  padding: 0.2rem 0;
+}
+.ref-label { color: var(--vp-c-text-2); }
+.ref-value { font-weight: 600; font-family: var(--vp-font-family-mono); }
+</style>