refactor(ct): migrate manual update backups to create_backup (batch 1)

Replace ad-hoc mv/cp .bak patterns with create_backup/restore_backup across
58 update scripts so failed updates can roll back via the shared ERR trap.

Co-authored-by: Cursor <cursoragent@cursor.com>

ct/flatnotes.sh

@@ -34,12 +34,10 @@ function update_script() {
     systemctl stop flatnotes
     msg_ok "Stopped Service"
 
-    msg_info "Backing up Configuration and Data"
-    cp /opt/flatnotes/.env /opt/flatnotes.env
-    cp -r /opt/flatnotes/data /opt/flatnotes_data_backup
-    msg_ok "Backed up Configuration and Data"
+    create_backup /opt/flatnotes/data
 
     fetch_and_deploy_gh_release "flatnotes" "dullage/flatnotes" "tarball"
+    restore_backup
 
     msg_info "Updating Flatnotes"
     cd /opt/flatnotes/client
@@ -52,12 +50,6 @@ function update_script() {
     $STD /usr/local/bin/uv sync
     msg_ok "Updated Flatnotes"
 
-    msg_info "Restoring Configuration and Data"
-    cp /opt/flatnotes.env /opt/flatnotes/.env
-    cp -r /opt/flatnotes_data_backup/. /opt/flatnotes/data
-    rm -f /opt/flatnotes.env
-    rm -r /opt/flatnotes_data_backup
-    msg_ok "Restored Configuration and Data"
 
     msg_info "Starting Service"
     systemctl start flatnotes