fix(security): restrict wildcard CORS origin in Flask API

[devprashant19]

Description

This PR resolves an issue where the Flask ML API was configured with a wildcard CORS policy (origins: "*"). Since the Flask API is strictly an internal service intended to be accessed only by the Node.js gateway, allowing any origin exposes internal endpoints (such as those requiring the X-Internal-Secret) to cross-origin attacks from any browser.
Closes #221

Changes

• Modified backend/api.py to replace the wildcard CORS configuration.
• The CORS origin is now restricted by the NODE_GATEWAY_ORIGIN environment variable, defaulting to http://localhost:3000.

Related Issues

• Resolves the wildcard CORS vulnerability.

Type of Change

• Security fix
• Bug fix
• New feature
• Refactoring

Pre-submission Checklist

• Tested locally to ensure cross-origin requests from unauthorized domains are blocked.
• Verified that internal requests from the Node.js gateway still succeed.

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