fix(server): enable clean URLs (/whitepapers/foo → foo.html)

Add extensions: ['html'] to express.static so the .html-less URLs resolve
to their matching files. Affects every static HTML route including
/whitepapers/distributive-equity, /whitepapers/eu-policy-brief, etc.

.html-suffixed URLs continue to work unchanged. Non-existent paths still
return the friendly 404 via the notFound middleware.

src/server.js

@@ -135,7 +135,9 @@ const rssRoutes = require('./routes/rss.routes');
 app.use('/', rssRoutes);
 
 // Static files
-app.use(express.static('public'));
+// extensions: ['html'] lets clean URLs (e.g. /whitepapers/eu-policy-brief) resolve
+// to the matching .html file. Without this, only the .html-suffixed URL works.
+app.use(express.static('public', { extensions: ['html'] }));
 
 // Health check endpoint (minimal, no sensitive data)
 app.get('/health', (req, res) => {