feat: nginx sec headers

[Lennart01]

Description

Changelog

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• The light- and dark-theme are both supported and tested
• The design was implemented and is responsive for all devices and screen sizes
• The application was tested in the most commonly used browsers (e.g. Chrome, Firefox, Safari)

(Optional) Visual Changes

[Schwehn42]

I'm usually a proponent of splitting stuff into multiple files, however I'm not sure in this case because it adds another file to the project root. Could you add in into the main file or if you want to split it maybe add a subdirectory?

[Lennart01]

Id argue this is fine.
Only one file

[Copilot]

Pull Request Overview

A PR to enhance nginx configuration with additional security headers and improve caching headers.

• Added multiple security headers: X-XSS-Protection, X-Content-Type-Options, Referrer-Policy, Content-Security-Policy, Permissions-Policy, X-Frame-Options, and Expect-CT.
• Updated the Cache-Control header for selected media files to include the "must-revalidate" directive.

[github-actions]

The deployment to the dev cluster was successful. You can find the deployment here: https://4397.development.scrumlr.fra.ics.inovex.io
This deployment is only for testing purposes and will be deleted after 1 week.
To redeploy rerun the workflow.
DO NOT STORE IMPORTANT DATA ON THIS DEPLOYMENT

Deployed Images

• ghcr.io/inovex/scrumlr.io/scrumlr-frontend:sha-fec2de7

• ghcr.io/inovex/scrumlr.io/scrumlr-server:sha-fec2de7

[octomind-dev]

🐙 Octomind

Test Report: 14/14 successful.

description	status	details
test all ways to open the setup flow	Passed ✅	click
sign-in	Passed ✅	click
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create lean coffee board	Passed ✅	click
create_and_delete_notes_and_actions_v2	Passed ✅	click
create and delete board columns	Passed ✅	click
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About Section Visibility Test	Passed ✅	click

commit sha: fec2de7