Sync local state with remote after untracked local evolution

Local site-ai/ had diverged significantly from the last commit pushed
to Gitea (new remote-env-setup/remove targets, SITE_CONFIG_DIR/MAIN_REPO
split, credential cleanup in server-install.sh, migration docs) without
ever being committed. This catches the repo up to what's actually on disk.

Also untracks the legacy user/ subtree left over from before content was
split into its own standalone repo (natascha-rieter.nl-user) — user/ is
gitignored here and stays a separate git repo, unaffected by this commit.

scripts/server-maintenance.sh

@@ -0,0 +1,29 @@
+#!/bin/bash
+set -e
+
+if [ "$#" -ne 2 ]; then
+    echo "Usage: server-maintenance.sh on|off <webroot>"
+    exit 1
+fi
+
+MODE="$1"
+WEBROOT="$2"
+CONFIG="$WEBROOT/user/config/system.yaml"
+
+if [ "$MODE" != "on" ] && [ "$MODE" != "off" ]; then
+    echo "Usage: server-maintenance.sh on|off <webroot>"
+    exit 1
+fi
+
+[ -f "$CONFIG" ] || { echo "Not found: $CONFIG"; exit 1; }
+
+VALUE="false"
+[ "$MODE" = "on" ] && VALUE="true"
+
+if grep -q "^[[:space:]]*offline:" "$CONFIG"; then
+    sed -i "s/^\([[:space:]]*\)offline: .*/\1offline: $VALUE/" "$CONFIG"
+else
+    printf '\npages:\n  offline: %s\n' "$VALUE" >> "$CONFIG"
+fi
+
+echo "Maintenance mode: $MODE (offline: $VALUE)"