news-digest: cache-bust style.css + app.js with generated_at timestamp

Browsers were serving stale frontend assets after rebuilds, which hid
the new world/local headline desks: the OLD app.js's refreshCounts()
only counted .item children (not .headline), so the new headline desks
came up with visibleItems=0 and got the .is-empty class which is
display:none. Hard refresh fixed it but only for the user who knew
to do that.

Append ?v=<generated_at strftime> to both link/script tags in
digest.html.j2 and archive.html.j2 so every digest run produces a new
asset URL. Works with the existing entrypoint.sh static-asset sync —
no other infra needed.

stacks/news-digest/templates/archive.html.j2

@@ -5,7 +5,7 @@
   <meta name="viewport" content="width=device-width,initial-scale=1" />
   <title>Daily Digest · Archive</title>
   <link rel="icon" type="image/svg+xml" href="favicon.svg" />
-  <link rel="stylesheet" href="style.css" />
+  <link rel="stylesheet" href="style.css?v={{ generated_at.strftime('%Y%m%d%H%M%S') }}" />
 </head>
 <body>
   <header class="masthead" role="banner">