Fold a conservative security-header set into the SAME opt-in proxy mode, so it's
zero-impact when off. When security.trust_reverse_proxy is on, an after_request
adds X-Content-Type-Options: nosniff, X-Frame-Options: SAMEORIGIN, and HSTS
(safe — only honoured over the proxy's HTTPS), via setdefault so it never clobbers
a header the proxy already set. No CSP (needs per-deploy tuning; better at the
proxy). When OFF (default), the after_request isn't registered → no headers added.
Tests: off adds none of the headers; on adds all three. Doc updated. 6 tests pass.
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