feat: add optional auth middleware

Made-with: Cursor

backend/middleware/optional-auth.js

@@ -0,0 +1,19 @@
+const jwt = require('jsonwebtoken');
+const { JWT_SECRET } = require('./auth');
+
+function optionalAuthenticateToken(req, res, next) {
+  const authHeader = req.headers['authorization'];
+  const token = authHeader && authHeader.split(' ')[1];
+
+  if (!token) {
+    req.user = null;
+    return next();
+  }
+
+  jwt.verify(token, JWT_SECRET, (err, user) => {
+    req.user = err ? null : user;
+    next();
+  });
+}
+
+module.exports = { optionalAuthenticateToken };