feat(docs): add interactive demo components for technical appendices

Add placeholder Vue components for visualizing technical concepts across multiple domains including frontend routing, browser rendering, cache design, queue design, database principles, API design, cloud services, and backend evolution. These components provide interactive educational content for the documentation.

Update documentation structure to include new appendix sections and enhance existing content with visual components. Remove unused 'codex' dependency from package.json.

docs/.vitepress/theme/components/appendix/database-intro/DatabaseRelationDemo.vue

@@ -0,0 +1,50 @@
+<template>
+  <div class="demo-container">
+    <div class="demo-header">
+      <h4>{{ title }}</h4>
+      <p class="hint">{{ description }}</p>
+    </div>
+    <div class="demo-content">
+      <el-alert type="info" :closable="false">
+        数据库关系演示组件占位符 - 待实现具体交互
+      </el-alert>
+    </div>
+  </div>
+</template>
+
+<script setup>
+import { ref } from 'vue'
+
+const title = ref('数据库关系演示')
+const description = ref('展示关系数据库中表与表之间的关系，包括一对一、一对多、多对多关系')
+</script>
+
+<style scoped>
+.demo-container {
+  border: 1px solid var(--vp-c-divider);
+  border-radius: 8px;
+  padding: 20px;
+  background: var(--vp-c-bg-soft);
+}
+
+.demo-header {
+  margin-bottom: 20px;
+}
+
+.demo-header h4 {
+  margin: 0 0 8px 0;
+  color: var(--vp-c-text-1);
+}
+
+.hint {
+  margin: 0;
+  font-size: 14px;
+  color: var(--vp-c-text-2);
+}
+
+.demo-content {
+  display: flex;
+  flex-direction: column;
+  gap: 16px;
+}
+</style>