🛡️ Sentinel: add security headers and structured logging to API template

[google-labs-jules]

This change enhances the security of the API Gateway template by adding standard security headers and improving logging practices.

Key improvements:

1. Security Headers: All API responses now include X-Content-Type-Options, X-Frame-Options, and Strict-Transport-Security headers to protect against common web vulnerabilities.
2. Structured Logging: Refactored error logs to use static messages and pass dynamic data (like item IDs) via the extra parameter, preventing log injection risks.
3. Validation: Added a new test case to ensure these security headers are correctly applied and maintained.

---

PR created automatically by Jules for task 13359687586114237140 started by @amrabed

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.