Proposal: Add Optional Content-Security-Policy (CSP) Header Support

Hi @AlemTuzlak,

I’d like to contribute to the project by adding support for a default `Content-Security-Policy` (CSP) header — turned **off by default**, but easy to enable when needed.

**What I propose:**

* Add CSP header support with a sensible default policy (e.g., `default-src 'none'`)
* Make it **opt-in** via an env variable or config flag

Happy to follow your preferred coding style or integration pattern. Let me know if you’re open to this — I can start working on a pull request right away.

Thanks!

Duncan

No files, tests, or entry points are named in the issue. Before work starts, clarify the policy, configuration mechanism, affected request path, and acceptance tests; what counts as done cannot be determined from the current proposal.

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