feat: add unfurl cards to blog page and homepage recent posts

Use two-strategy approach to work around async shortcode limitation
in deeply nested Nunjucks includes:

- blog.njk: async {% unfurl %} shortcode (top-level, works fine)
- recent-posts.njk: sync {{ url | unfurlCard | safe }} filter
  (reads from pre-populated disk cache)

eleventy.before hook scans content files and pre-fetches all
interaction URLs before templates render, ensuring the sync filter
always has data — even on first build.

lib/unfurl-shortcode.js

@@ -35,7 +35,7 @@ function getCachePath(url) {
   return resolve(CACHE_DIR, `${hash}.json`);
 }
 
-function readCache(url) {
+export function readCache(url) {
   const path = getCachePath(url);
   if (!existsSync(path)) return undefined; // undefined = not cached
   try {
@@ -57,7 +57,7 @@ function writeCache(url, metadata, failed = false) {
   writeFileSync(path, JSON.stringify({ cachedAt: Date.now(), metadata, failed }));
 }
 
-function extractDomain(url) {
+export function extractDomain(url) {
   try {
     return new URL(url).hostname.replace(/^www\./, "");
   } catch {
@@ -65,7 +65,7 @@ function extractDomain(url) {
   }
 }
 
-function escapeHtml(str) {
+export function escapeHtml(str) {
   if (!str) return "";
   return str
     .replace(/&/g, "&")
@@ -74,12 +74,12 @@ function escapeHtml(str) {
     .replace(/"/g, """);
 }
 
-function renderFallbackLink(url) {
+export function renderFallbackLink(url) {
   const domain = escapeHtml(extractDomain(url));
   return `<a href="${escapeHtml(url)}" rel="noopener" target="_blank">${domain}</a>`;
 }
 
-function renderCard(url, metadata) {
+export function renderCard(url, metadata) {
   const og = metadata.open_graph || {};
   const tc = metadata.twitter_card || {};
 
@@ -117,41 +117,58 @@ function renderCard(url, metadata) {
 </div>`;
 }
 
+/**
+ * Fetch unfurl metadata for a URL and populate the disk cache.
+ * Returns the rendered HTML card (or fallback link on failure).
+ */
+export async function prefetchUrl(url) {
+  if (!url) return "";
+
+  // Already cached — skip network fetch
+  const cached = readCache(url);
+  if (cached !== undefined) {
+    return cached.failed ? renderFallbackLink(url) : renderCard(url, cached.metadata);
+  }
+
+  const metadata = await throttled(async () => {
+    try {
+      return await unfurl(url, {
+        timeout: 20000,
+        headers: { "User-Agent": USER_AGENT },
+      });
+    } catch (err) {
+      console.warn(`[unfurl] Failed to fetch ${url}: ${err.message}`);
+      return null;
+    }
+  });
+
+  if (!metadata) {
+    writeCache(url, null, true);
+    return renderFallbackLink(url);
+  }
+
+  writeCache(url, metadata, false);
+  return renderCard(url, metadata);
+}
+
+/**
+ * Synchronous cache-only lookup. Returns the rendered card HTML if cached,
+ * a fallback link if cached as failed, or empty string if not cached.
+ * Safe to use in deeply nested Nunjucks includes where async isn't supported.
+ */
+export function getCachedCard(url) {
+  if (!url) return "";
+  const cached = readCache(url);
+  if (cached === undefined) return renderFallbackLink(url);
+  if (cached.failed) return renderFallbackLink(url);
+  return renderCard(url, cached.metadata);
+}
+
 /**
  * Register the {% unfurl "URL" %} shortcode on an Eleventy config.
  */
 export default function registerUnfurlShortcode(eleventyConfig) {
   eleventyConfig.addAsyncShortcode("unfurl", async function (url) {
-    if (!url) return "";
-
-    // Check cache first (returns undefined if not cached)
-    const cached = readCache(url);
-    if (cached !== undefined) {
-      // Cached failure → render fallback link
-      if (cached.failed) return renderFallbackLink(url);
-      // Cached success → render card
-      return renderCard(url, cached.metadata);
-    }
-
-    // Not cached — fetch with concurrency limiting
-    const metadata = await throttled(async () => {
-      try {
-        return await unfurl(url, {
-          timeout: 20000,
-          headers: { "User-Agent": USER_AGENT },
-        });
-      } catch (err) {
-        console.warn(`[unfurl] Failed to fetch ${url}: ${err.message}`);
-        return null;
-      }
-    });
-
-    if (!metadata) {
-      writeCache(url, null, true); // cache failure for 1 day
-      return renderFallbackLink(url);
-    }
-
-    writeCache(url, metadata, false);
-    return renderCard(url, metadata);
+    return prefetchUrl(url);
   });
 }