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

backend/src/utils/tokens.ts

@@ -20,3 +20,7 @@ export function verifyToken(token: string): JwtPayload {
 export function generateVerificationToken(): string {
   return crypto.randomBytes(32).toString('hex')
 }
+
+export function hashToken(token: string): string {
+  return crypto.createHash('sha256').update(token).digest('hex')
+}