fix: exempt public unauthenticated endpoints from CSRF protection

The lusca CSRF implementation was breaking login and registration because
the frontend doesn't fetch or send CSRF tokens. This is a structural issue
that requires frontend implementation.

As a pragmatic fix, this commit exempts public unauthenticated endpoints
from CSRF protection:
- /api/login, /api/register, /api/verify-email
- /api/version, /api/registration-status, /api/health
- /api/oidc/* (all OIDC authentication endpoints)
- /api/feature-flags

Authenticated endpoints still require CSRF tokens via lusca.

Also updates csrf.js to use lusca's token generation mechanism, making
it compatible with the global lusca CSRF middleware.

TODO: Implement proper CSRF token handling in the frontend for enhanced
security on public endpoints.

backend/app.js

@@ -97,10 +97,28 @@ const lusca = require('lusca');
 
 // Pre-check middleware to exempt test/Bearer requests before lusca runs
 app.use((req, res, next) => {
+    // Public unauthenticated endpoints that should bypass CSRF
+    // These are routes that don't require authentication and are used during login/registration
+    const publicPaths = [
+        '/api/login',
+        '/api/register',
+        '/api/verify-email',
+        '/api/version',
+        '/api/registration-status',
+        '/api/health',
+    ];
+
+    const isPublicPath = publicPaths.some((path) => req.path === path);
+    const isOidcPath = req.path.startsWith('/api/oidc/');
+    const isFeatureFlagsPath = req.path.startsWith('/api/feature-flags');
+
     // Mark exempt requests so lusca wrapper can skip them
     if (
         process.env.NODE_ENV === 'test' ||
-        req.headers.authorization?.startsWith('Bearer ')
+        req.headers.authorization?.startsWith('Bearer ') ||
+        isPublicPath ||
+        isOidcPath ||
+        isFeatureFlagsPath
     ) {
         req._csrfExempt = true;
     }

backend/middleware/csrf.js

@@ -1,11 +1,16 @@
-const csurf = require('@dr.pogodin/csurf');
+const lusca = require('lusca');
 
-const csrfMiddleware = csurf({
-    cookie: false,
-    value: (req) => {
-        return req.headers['x-csrf-token'] || req.body?._csrf;
-    },
-});
+const csrfMiddleware = (req, res, next) => {
+    if (!req.session) {
+        return res.status(500).json({ error: 'Session not initialized' });
+    }
+
+    if (!req.session._csrf) {
+        req.session._csrf = require('crypto').randomBytes(16).toString('hex');
+    }
+
+    next();
+};
 
 const csrfProtection = (req, res, next) => {
     if (
@@ -16,11 +21,22 @@ const csrfProtection = (req, res, next) => {
         return next();
     }
 
-    return csrfMiddleware(req, res, next);
+    return lusca.csrf({
+        header: 'x-csrf-token',
+        cookie: false,
+    })(req, res, next);
 };
 
 const generateToken = (req) => {
-    return req.csrfToken ? req.csrfToken() : '';
+    if (!req.session) {
+        return '';
+    }
+
+    if (!req.session._csrf) {
+        req.session._csrf = require('crypto').randomBytes(16).toString('hex');
+    }
+
+    return req.session._csrf;
 };
 
 module.exports = {