feat: Enhance security and validation in backend

- Added helmet for security headers and configured content security policy
- Implemented CORS with a whitelist for allowed origins
- Introduced express-validator for input validation in API endpoints
- Set request size limits to prevent DoS attacks
- Added global error handling and 404 response
- Updated TypeScript configuration to use node16 module resolution
- Improved Docker Compose configuration for security and resource limits
- Created a comprehensive .env.example for environment configuration
- Implemented automated security scans in CI/CD with Trivy
- Added cleanup script for debugging ports
- Established a detailed security policy document

backend/package.json

@@ -24,6 +24,8 @@
   },
   "dependencies": {
     "cors": "^2.8.5",
-    "express": "^4.18.3"
+    "express": "^4.18.3",
+    "helmet": "^7.1.0",
+    "express-validator": "^7.0.1"
   }
 }