fix: harden unfurl timeout + persist CI fetch cache

- Add hard 22s Promise.race deadline in prefetchUrl() to guard against
  TCP-level connection hangs that bypass unfurl.js's read-only timeout.
  Fixes builds hanging indefinitely on unresponsive hosts.

- Add actions/cache step to deploy.yml persisting .cache/ between runs.
  Prevents webmention (and all eleventy-fetch) data loss on transient
  502s: a populated cache means failures return existing data, not [].

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/deploy.yml

@@ -32,6 +32,14 @@ jobs:
             "sudo bastille cmd node cat /usr/local/indiekit/content/.indiekit/homepage.json" \
             > content/.indiekit/homepage.json
             
+      - name: Cache Eleventy fetch cache
+        uses: actions/cache@v4
+        with:
+          path: .cache
+          key: eleventy-fetch-${{ runner.os }}-${{ hashFiles('package-lock.json') }}
+          restore-keys: |
+            eleventy-fetch-${{ runner.os }}-
+
       - name: Build CSS
         run: npm run build:css