feat: enhance security with token blacklisting and input sanitization; add helmet for security headers

backend/package-lock.json

@@ -14,6 +14,7 @@
         "dotenv": "^16.4.7",
         "express": "^5.1.0",
         "express-rate-limit": "^7.5.0",
+        "helmet": "^8.1.0",
         "jsonwebtoken": "^9.0.2",
         "nodemailer": "^7.0.3",
         "zod": "^3.24.2"
@@ -1862,6 +1863,15 @@
         "node": ">= 0.4"
       }
     },
+    "node_modules/helmet": {
+      "version": "8.1.0",
+      "resolved": "https://registry.npmjs.org/helmet/-/helmet-8.1.0.tgz",
+      "integrity": "sha512-jOiHyAZsmnr8LqoPGmCjYAaiuWwjAPLgY8ZX2XrmHawt99/u1y6RgrZMTeoPfpUbV96HOalYgz1qzkRbw54Pmg==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=18.0.0"
+      }
+    },
     "node_modules/http-errors": {
       "version": "2.0.1",
       "resolved": "https://registry.npmjs.org/http-errors/-/http-errors-2.0.1.tgz",