fix nginx security headers not being applied to https responses

nginx add_header inheritance meant X-Content-Type-Options and X-Frame-Options
were silently dropped from all https server blocks. moved all security headers
into a shared snippet (security-headers.inc) included per server block. added
pytest-based header verification that auto-discovers sites from conf files.

docker/nginx/conf.d/security-headers.inc

@@ -0,0 +1,5 @@
+add_header Strict-Transport-Security "max-age=63072000; preload" always;
+add_header X-Content-Type-Options "nosniff" always;
+add_header X-Frame-Options "DENY" always;
+add_header Referrer-Policy "strict-origin-when-cross-origin" always;
+add_header Permissions-Policy "geolocation=(), microphone=(), camera=()" always;