fix(security): add Helmet headers and rate limiting on auth endpoints

Helmet adds HTTP security headers (HSTS, X-Frame-Options,
X-Content-Type-Options, etc.). CSP is disabled pending deployment-
specific tuning. Rate limiting (10 req / 15 min per IP) is applied
to POST /login and POST / (registration) to mitigate brute force
and credential stuffing attacks.

src/routes/api/cms/users.routes.js

@@ -1,4 +1,5 @@
 const express = require('express');
+const rateLimit = require('express-rate-limit');
 const {
     authenticate, createUser, getUser, loginAsUser,
     updateUser, updateUserEmail, updateUserPassword, updateUserRole, updateUserUsername
@@ -7,11 +8,19 @@ const auth = require('../../../middlewares/auth');
 
 const userRouter = express.Router();
 
+const authLimiter = rateLimit({
+    windowMs: 15 * 60 * 1000,
+    max: 10,
+    standardHeaders: true,
+    legacyHeaders: false,
+    message: { errors: { 'rate limit': 'too many attempts, please try again later' } }
+});
+
 userRouter.get('/', auth.required, getUser);
 userRouter.put('/', auth.required, updateUser);
-userRouter.post('/', auth.optional, createUser);
+userRouter.post('/', authLimiter, auth.optional, createUser);
 userRouter.get('/authenticate', auth.optional, authenticate);
-userRouter.post('/login', auth.optional, loginAsUser);
+userRouter.post('/login', authLimiter, auth.optional, loginAsUser);
 //userRouter.post('/register', auth.optional, createUser);
 //userRouter.get('/users', auth.required, getAllUsers);
 userRouter.put('/update/email', auth.required, updateUserEmail);