Fix Express 5 404 handler: Use middleware pattern instead of route

Express 5 doesn't support wildcard routes with app.all('*').
Changed to standard middleware pattern: app.use((req, res) => {...})

backend/src/index.ts

@@ -133,8 +133,8 @@ app.use('/api/documents', documentRoutes);
 app.use('/api/subscriptions', subscriptionRoutes);
 app.use('/api/notifications', notificationRoutes);
 
-// 404 handler - Express 5 requires explicit path
-app.all('*', (req, res) => {
+// 404 handler - Express 5 compatible
+app.use((req, res) => {
   res.status(404).json({
     success: false,
     message: 'Route not found',