nginx: security headers on every response (closes #18)

nginx sent none of the security headers REPO_POLICIES.md requires.
security-headers.conf now sets all six with always, included at server
level and again in /assets/, whose own add_header would otherwise drop
them. nginx hides the copies netwatch-server sets, so /api/ and the
health check carry each header once. The content security policy
allows no inline script or style; the host row's status dot took its
grey from a style attribute, now a class. connect-src is * because
several probed hosts redirect to other hosts and the browser checks
every redirect against it. Referrer-Policy is no-referrer, as the
backend already sends.

Model: opus-5-5

nginx.conf

@@ -8,6 +8,11 @@ server {
     # Keep the nginx version out of the Server header and error pages.
     server_tokens off;
 
+    # The security headers, on every response. An add_header in a
+    # location drops every add_header from here, so a location with one
+    # of its own includes this file again.
+    include /etc/nginx/security-headers.conf;
+
     root /usr/share/nginx/html;
     index index.html;
 
@@ -32,6 +37,7 @@ server {
     location /assets/ {
         expires 1y;
         add_header Cache-Control "public, immutable";
+        include /etc/nginx/security-headers.conf;
     }
 
     # netwatch-server, the Go backend, runs in the same container and
@@ -45,6 +51,16 @@ server {
     proxy_set_header X-Forwarded-For $remote_addr;
     proxy_set_header X-Forwarded-Proto $scheme;
 
+    # netwatch-server sets the same security headers on its own
+    # responses. Its copies are dropped so that each header goes out
+    # once, as security-headers.conf sets it.
+    proxy_hide_header Strict-Transport-Security;
+    proxy_hide_header Content-Security-Policy;
+    proxy_hide_header X-Frame-Options;
+    proxy_hide_header X-Content-Type-Options;
+    proxy_hide_header Referrer-Policy;
+    proxy_hide_header Permissions-Policy;
+
     location /api/ {
         proxy_pass http://127.0.0.1:8081;
     }