Introduce image and CSS CSP controls

[Zhey-on]

Summary

This PR introduces CSP controls for image and CSS sources.

In line with the issue intent, the defaults are kept relatively permissive to prevent breaking changes on existing instances, while still providing clear options to tighten policies where needed.

Changes

• Added img-src and style-src directives to CSP handling.
• Added environment/config options:
  • ALLOWED_IMAGE_SOURCES
  • ALLOWED_CSS_SOURCES
• Kept permissive defaults when these options are not set, to reduce rollout risk.
• Added tests in SecurityHeaderTest for:
  • default behavior
  • custom override behavior
• Added documentation:
  • .env.example.complete entries and examples
  • development docs section describing controls and hardening guidance
  • README pointer to the CSP docs section

Why this approach

The implementation is intentionally conservative by default, especially compared to JS/iframe controls, so instances do not unexpectedly break after upgrade. At the same time, admins can now explicitly restrict allowed sources as part of their hardening process.

Testing

• docker compose run --rm app php artisan test tests/SecurityHeaderTest.php --filter="style src|img src|csp"

Closes #6033.