Security Headers

[nensii21]

Description

Harden backend security.

Implement
CSP
HSTS
Rate Limiting
CORS
Secure Cookies
Helmet-equivalent headers

Acceptance Criteria
Security scan passes
Documentation included

[ayeshafaeza-2427]

Hi! I'd like to work on this issue as part of ECSoC 2026.
Could you please assign it to me? Thank you!