fix: secure express API templates with helmet and cors

[mramansayyad]

Description of Changes

This PR addresses security concerns for Express API templates generated by Celtrix.

Specifically, this PR:

1. Adds helmet and cors middlewares to the base server templates:
   • templates/mean/server/server.js
   • templates/mern/server/server.js
   • templates/mevn/server/server.js
2. This ensures that any generated Express APIs automatically start with production-ready security headers (like XSS protection, clickjacking prevention, CORS restrictions, etc.) out-of-the-box.

Related Issues

Fixes #247

Verification

• Verified the templates compile successfully and contain the correct require statements and middleware setup.

GSSoC '26

• Yes, I am participating in GirlScript Summer of Code and would like to build this.