fix: use proper User-Agent for unfurl requests

The default facebookexternalhit UA causes many sites to block or
redirect-loop, resulting in timeouts. Switch to a well-identified
bot UA that sites handle correctly.

lib/unfurl-shortcode.js

@@ -6,6 +6,7 @@ import { createHash } from "crypto";
 const CACHE_DIR = resolve(import.meta.dirname, "..", ".cache", "unfurl");
 const CACHE_DURATION_MS = 7 * 24 * 60 * 60 * 1000; // 1 week
 const FAILURE_CACHE_MS = 24 * 60 * 60 * 1000; // 1 day for failed fetches
+const USER_AGENT = "Mozilla/5.0 (compatible; Indiekit/1.0; +https://getindiekit.com)";
 
 // Concurrency limiter — prevents overwhelming outbound network
 let activeRequests = 0;
@@ -135,7 +136,10 @@ export default function registerUnfurlShortcode(eleventyConfig) {
     // Not cached — fetch with concurrency limiting
     const metadata = await throttled(async () => {
       try {
-        return await unfurl(url, { timeout: 20000 });
+        return await unfurl(url, {
+          timeout: 20000,
+          headers: { "User-Agent": USER_AGENT },
+        });
       } catch (err) {
         console.warn(`[unfurl] Failed to fetch ${url}: ${err.message}`);
         return null;