Uses sqlite3.Connection.backup for an online, WAL-safe copy. Resolves
the DB path from QUARTERMASTER_DB_URL or ./quartermaster.db, places
the snapshot in <db-dir>/backups (override with QUARTERMASTER_BACKUP_DIR),
timestamps the filename, and tags it with an optional reason slug.
Absent DB file is a soft exit so the script is safe to call from hooks.
Refs #5
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
