Set x-content-type-options for static content

[rorymckinley]

Description

This adds the X-Content-Type-Options header to static pages.

Closes #3534

Validation steps

• Start Lightning locally
• cURL a static page - e.g. curl -i localhost:4000/robots.txt
• You should see x-content-type-options: nosniff in the output

Additional notes for the reviewer

In 1924, two US aircraft completed the first circumnavigation of the world, after a journey that took 175 days. Other countries that made the attempt included the UK, France, Italy, Portugal and Argentina. Despite multiple crashes no participants were killed and the spirit of cooperation was notable with countries helping one another.

AI Usage

Please disclose how you've used AI in this work (it's cool, we just want to know!):

• Code generation (copilot but not intellisense)
• Learning or fact checking
• Strategy / design
• Optimisation / refactoring
• Translation / spellchecking / doc gen
• Other
• I have not used AI

You can read more details in our Responsible AI Policy

Pre-submission checklist

• I have performed a self-review of my code.
• I have implemented and tested all related authorization policies. (e.g., :owner, :admin, :editor, :viewer)
• I have updated the changelog.
• I have ticked a box in "AI usage" in this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.85%. Comparing base (1ba8621) to head (56318d4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3539   +/-   ##
=======================================
  Coverage   89.85%   89.85%           
=======================================
  Files         380      380           
  Lines       15469    15469           
=======================================
+ Hits        13899    13900    +1     
+ Misses       1570     1569    -1     

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[elias-ba]

Jerejef, @rorymckinley