feat(docs): add comprehensive SEO optimization for static docs site

Add per-page metadata, Open Graph / Twitter tags, canonical URLs,
hreflang, sitemap generation, robots.txt, and JSON-LD structured data
for the bilingual (en/cn) documentation site.

docs/lib/metadata.ts

@@ -0,0 +1,75 @@
+import type { Metadata } from 'next';
+
+export const SITE_URL = 'https://dbxio.com';
+export const SITE_NAME = 'DBX';
+export const DEFAULT_DESCRIPTION =
+  '25+ databases in 15 MB. Desktop & Docker self-hosting, with built-in AI assistant.';
+export const DEFAULT_OG_IMAGE = '/logo.png';
+
+const LOCALE_MAP: Record<string, string> = {
+  en: 'en_US',
+  cn: 'zh_CN',
+};
+
+const HTML_LANG_MAP: Record<string, string> = {
+  en: 'en',
+  cn: 'zh-CN',
+};
+
+export function getHtmlLang(lang: string): string {
+  return HTML_LANG_MAP[lang] ?? 'en';
+}
+
+function swapLang(path: string, to: string): string {
+  return path.replace(/^\/(en|cn)/, `/${to}`);
+}
+
+interface BuildMetadataParams {
+  title: string;
+  description: string;
+  path: string;
+  lang: string;
+  ogType?: 'website' | 'article';
+  images?: string[];
+  lastModified?: Date;
+}
+
+export function buildMetadata({
+  title,
+  description,
+  path,
+  lang,
+  ogType = 'website',
+  images,
+}: BuildMetadataParams): Metadata {
+  const canonical = `${SITE_URL}${path}`;
+  const locale = LOCALE_MAP[lang] ?? 'en_US';
+
+  return {
+    title,
+    description,
+    alternates: {
+      canonical,
+      languages: {
+        en: `${SITE_URL}${swapLang(path, 'en')}`,
+        zh: `${SITE_URL}${swapLang(path, 'cn')}`,
+        'x-default': `${SITE_URL}${swapLang(path, 'en')}`,
+      },
+    },
+    openGraph: {
+      title,
+      description,
+      url: canonical,
+      siteName: SITE_NAME,
+      type: ogType,
+      locale,
+      images: images?.map((url) => ({ url })) ?? [{ url: DEFAULT_OG_IMAGE }],
+    },
+    twitter: {
+      card: 'summary_large_image',
+      title,
+      description,
+      images: images ?? [DEFAULT_OG_IMAGE],
+    },
+  };
+}