fix(backup): address code review feedback - accessibility, CSRF, docs, imports

- Move `import os` to top-level in app/views/backup.py
- Fix docstring in BackupRecord model to remove non-existent 'location' field
- Replace browser confirm() dialogs with accessible modal dialog (role=dialog, aria-modal, aria-labelledby)
- Add csrfToken() helper that validates token presence instead of silently falling back to empty string
- Fix aria-live region to remain in DOM (screen-reader friendly) rather than using x-show
- Add Backup & Restore section to docs/ConfigurationGuide.md with retention table
- Add backup env vars to .env.demo with comments

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

.env.demo

@@ -350,6 +350,20 @@ WEBHOOK_ENABLED=True
 UPTIME_KUMA_URL=https://status.example.com/api/push/abcdef123456?status=up
 UPTIME_KUMA_PING_INTERVAL=5
 
+# Backup & Restore
+# Enable automatic scheduled backups (hourly, daily, weekly)
+BACKUP_ENABLED=True
+# Directory for local backup archives (defaults to <WORKDIR>/backups)
+# BACKUP_DIR=/data/backups
+# Optional remote destination: s3, dropbox, google_drive, onedrive, nextcloud, webdav, ftp, sftp, email
+# BACKUP_REMOTE_DESTINATION=s3
+# Sub-folder used when uploading backup archives to the remote destination
+BACKUP_REMOTE_FOLDER=backups
+# Retention: number of snapshots to keep per tier
+BACKUP_RETAIN_HOURLY=96   # 4 days of hourly snapshots
+BACKUP_RETAIN_DAILY=21    # 3 weeks of daily snapshots
+BACKUP_RETAIN_WEEKLY=13   # ~3 months of weekly snapshots
+
 # **Full-Text Search (Meilisearch)**
 # URL for the Meilisearch instance.
 # Default is "http://meilisearch:7700" — the Docker Compose / K8s service name —