feat: add sitemap generation for SEO enhancement

Implements dynamic sitemap generation to improve SEO by providing search engines with updated site structure and content availability. This includes a root sitemap index and paginated sitemap files for individual posts, enhancing crawl efficiency and content discoverability.

public/robots.txt

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+User-agent: *
+
+Sitemap: /sitemap.xml

src/pages/sitemap.xml.js

@@ -0,0 +1,36 @@
+import { getChannelInfo } from '../lib/telegram'
+
+export const prerender = false
+
+export async function GET(Astro) {
+  const request = Astro.request
+  const url = new URL(request.url)
+  const channel = await getChannelInfo(Astro)
+  const posts = channel.posts || []
+
+  const pageSize = 20
+  let count = +posts[0]?.id
+
+  const pages = []
+  pages.push(count)
+  while (count > pageSize) {
+    count -= pageSize
+    pages.push(count)
+  }
+
+  const sitemaps = pages.map((page) => {
+    return `
+<sitemap>
+  <loc>${url.origin}/sitemap/${page}.xml</loc>
+</sitemap>`
+  })
+
+  return new Response(`<?xml version="1.0" encoding="UTF-8"?>
+<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+  ${sitemaps.join('')}
+</sitemapindex>`, {
+    headers: {
+      'Content-Type': 'application/xml',
+    },
+  })
+}

src/pages/sitemap/[cursor].xml.js

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+import { getChannelInfo } from '../../lib/telegram'
+
+export const prerender = false
+
+export async function GET(Astro) {
+  const request = Astro.request
+  const url = new URL(request.url)
+  const channel = await getChannelInfo(Astro, {
+    before: Astro.params.cursor,
+  })
+  const posts = channel.posts || []
+
+  const xmlUrls = posts.map(post => `
+    <url>
+      <loc>${url.origin}/posts/${post.id}</loc>
+      <lastmod>${new Date(post.datetime).toISOString()}</lastmod>
+    </url>
+  `).join('')
+
+  return new Response(`<?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+  ${xmlUrls}
+</urlset>`, {
+    headers: {
+      'Content-Type': 'application/xml',
+    },
+  })
+}