fix: secure unauthenticated ML endpoints (#537)

[Suyash2527]

What does this PR do?

This PR secures three resource-intensive routes (/comparison, /api/analyze, and /api/explain) that were missing authentication decorators, preventing unauthenticated users from bypassing login and consuming GPU/CPU time and AI API quota.

Related Issue

Closes #537

Changes made

• Created an @api_login_required decorator that returns a proper JSON 401 Unauthorized response for unauthenticated API requests instead of an HTML redirect.
• Protected the /comparison route using the standard Flask-Login @login_required decorator.
• Protected the /api/analyze and /api/explain routes using the new @api_login_required decorator.

Checklist

• I have tested my changes
• My code follows project guidelines
• No new errors introduced

[github-actions]

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