feat/security headers

[Aharshi3614]

Closes #229

Added security headers using Helmet to protect against common web vulnerabilities.

Changes:

• Installed helmet package
• Added app.use(helmet()) middleware
• Security headers now added to all responses

Headers Added:

Header	Purpose
X-XSS-Protection	Prevents XSS attacks
X-Frame-Options	Prevents clickjacking
X-Content-Type-Options	Prevents MIME sniffing
Content-Security-Policy	Controls allowed resources

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