CI: Fix lastmod property of articles

We use the git commit to determine the lastmod date, but the checkout action uses a shallow clone by default.

.forgejo/workflows/deploy.yaml

@@ -30,6 +30,8 @@ jobs:
           apk add rsync openssh
 
       - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0 # pull full history for page lastmod by git commit date
 
       - name: Patch baseURL (staging only)
         if: github.ref_name != 'main'