fix: auto-invalidate cache on version update — closes #121

- Add version check on app startup: compare server version with stored
  client version, clear all SW caches and reload on mismatch
- Set Cache-Control: no-cache on index.html so browsers always fetch
  the latest version instead of serving stale cached HTML

server/src/index.ts

@@ -163,8 +163,16 @@ app.use('/api/backup', backupRoutes);
 // Serve static files in production
 if (process.env.NODE_ENV === 'production') {
   const publicPath = path.join(__dirname, '../public');
-  app.use(express.static(publicPath));
+  app.use(express.static(publicPath, {
+    setHeaders: (res, filePath) => {
+      // Never cache index.html so version updates are picked up immediately
+      if (filePath.endsWith('index.html')) {
+        res.setHeader('Cache-Control', 'no-cache, no-store, must-revalidate');
+      }
+    },
+  }));
   app.get('*', (req: Request, res: Response) => {
+    res.setHeader('Cache-Control', 'no-cache, no-store, must-revalidate');
     res.sendFile(path.join(publicPath, 'index.html'));
   });
 }