fix: cache-bust static asset URLs (?v=2)

Browsers cached the pre-fix 404 responses for /static/js/markdown.js and
/static/css/markdown.css (responses carry Cache-Control max-age=14400),
so users kept seeing broken pages after redeploy. A versioned query
string forces clients to refetch the assets; bump ?v= whenever the
static files change.

templates/layouts/base.html

@@ -111,13 +111,16 @@
 {{define "markdown_assets"}}
 {{/* Markdown rendering assets: marked (pinned UMD build), DOMPurify,
     highlight.js and the shared renderer + styles. Include on any page that
-    renders Markdown (articles, comments, editor previews). */}}
+    renders Markdown (articles, comments, editor previews).
+    Local static files carry a ?v= cache-buster: bump it whenever
+    static/js/markdown.js or static/css/markdown.css changes, otherwise
+    browsers may keep serving stale cached copies. */}}
 <link rel="stylesheet" href="https://cdn.jsdelivr.net/gh/highlightjs/cdn-release@11.12.0/build/styles/github.min.css">
-<link rel="stylesheet" href="/static/css/markdown.css">
+<link rel="stylesheet" href="/static/css/markdown.css?v=2">
 <script src="https://cdn.jsdelivr.net/npm/marked@15.0.12/marked.min.js"></script>
 <script src="https://cdn.jsdelivr.net/npm/dompurify@3.4.13/dist/purify.min.js"></script>
 <script src="https://cdn.jsdelivr.net/gh/highlightjs/cdn-release@11.12.0/build/highlight.min.js"></script>
-<script src="/static/js/markdown.js"></script>
+<script src="/static/js/markdown.js?v=2"></script>
 {{end}}
 
 {{define "footer"}}