scripts/migrate_to_systemd.sh: SP_SKIP_NGINX flag

For hosts where nginx + /dashboard + certbot-via-nginx are already
configured correctly AND the existing vhost has custom directives
that must NOT be overwritten (e.g. support.ardupilot.org's
^/$ -> ardupilot.org/dev/docs/support_proxy.html redirect), pass
SP_SKIP_NGINX=1 to run only the webui.json + systemd + verify
steps. Skips apache stop, nginx apt install, vhost write, landing
page write, nginx -t/reload, and the certbot renewer migration.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: tridge

scripts/migrate_to_systemd.sh

@@ -19,6 +19,16 @@
 #                        away. Re-run with this flag once the new
 #                        stack has bedded in for a day or two.
 #
+#   SP_SKIP_NGINX=1    — skip everything web-server-related (steps
+#                        1-6: apache stop, nginx install, vhost
+#                        write, landing page, nginx reload, certbot
+#                        renewer migration). Use on hosts where
+#                        nginx + /dashboard are already configured
+#                        correctly AND have a custom vhost (e.g. a
+#                        root redirect) that must be preserved.
+#                        Run only the webui.json / systemd / verify
+#                        steps.
+#
 # What it does:
 #   1. Sanity-check the user, home dir, source dir, data dir, cert dir.
 #   2. Stop + disable apache2 (preserves binary + config for rollback).
@@ -62,6 +72,10 @@ SUDO=
 echo "===== migrate $SP_DOMAIN (user=$SP_USER home=$HOMEDIR) ====="
 echo
 
+if [ "${SP_SKIP_NGINX:-}" = "1" ]; then
+echo "--- SP_SKIP_NGINX=1: skipping steps 1-6 (apache stop, nginx install, vhost, landing page, reload, certbot renewer migration) ---"
+else
+
 # ----- 1. Stop + disable apache2 -------------------------------------
 echo "--- step 1/9: stop and disable apache2 (preserved on disk for rollback) ---"
 if systemctl is-enabled apache2 >/dev/null 2>&1 \
@@ -186,6 +200,8 @@ else
     echo "no $RENEW_CONF — cert may have been issued differently; check certbot manually"
 fi
 
+fi   # end of SP_SKIP_NGINX guard
+
 # ----- 7. webui.json -------------------------------------------------
 echo
 echo "--- step 7/9: update $DATA/webui.json (host=127.0.0.1 + behind_proxy) ---"