deploy: fix nginx security headers dropped by add_header inheritance

Found while verifying the first real provision: none of the four
security headers reached the browser on any HTML page.

nginx's add_header inheritance is all-or-nothing — a location that sets
any add_header of its own discards every header inherited from the
server block. "/" resolves through try_files to `location = /index.html`,
which sets Cache-Control, so HSTS, X-Content-Type-Options,
X-Frame-Options and Referrer-Policy were silently dropped exactly where
they matter. /assets/ lost them the same way.

Move the four into snippets/security-headers.conf and include it in the
server block and in both locations that add a header of their own.

Also from the same provision run:
- ssl_stapling is dead config now that Let's Encrypt certificates carry
  no OCSP responder URL; it only logs a warning per cert on each reload;
- README step 3 chmod'ed /etc/exo to 750 but never set its group, so the
  exo user could not traverse it and exo-deploy died on "cannot read
  /etc/exo/<app>.env";
- README step 7 dropped http-extras.conf into conf.d without disabling
  the same directives in Debian's stock nginx.conf, and nginx refuses to
  start on a duplicate gzip / server_tokens.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

deploy/nginx/snippets/ssl-params.conf

@@ -4,8 +4,10 @@ ssl_prefer_server_ciphers off;
 ssl_session_cache shared:SSL:10m;
 ssl_session_timeout 1d;
 ssl_session_tickets off;
-ssl_stapling on;
-ssl_stapling_verify on;
+# Let's Encrypt stopped putting an OCSP responder URL in its certificates, so
+# stapling is a no-op that only produces a warning per cert on every reload.
+# ssl_stapling on;
+# ssl_stapling_verify on;
 # Yandex DNS — reachable from Russian datacentres, unlike 8.8.8.8 at times.
 resolver 77.88.8.8 77.88.8.1 valid=300s;
 resolver_timeout 5s;