Add scheduled backups of database and uploads

The backend now writes a timestamped tar.gz of the SQLite snapshot and all
per-order files to BACKUP_DIR on a cron schedule (default 03:00), keeps the
newest BACKUP_KEEP archives, and catches up once on start if a slot was
missed. Configurable via BACKUP_CRON / BACKUP_KEEP / BACKUP_DIR / TZ.
Includes user-facing restore docs in docs/backups.md.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.gitignore

@@ -7,4 +7,5 @@ __pycache__/
 .venv/
 .env
 .DS_Store
-.data/
+.data/
+.backups/