Fix Express 5 breaking change: Replace app.use('*') with app.all('*')

Express 5 no longer supports wildcard '*' in app.use() for 404 handlers.
Changed to app.all('*') which is the correct Express 5 pattern.

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
+// 404 handler - Express 5 requires explicit path
-app.use('*', (req, res) => {
+app.all('*', (req, res) => {
   res.status(404).json({
     success: false,
     message: 'Route not found',