Take a consistent, atomic copy of the DB at lifespan startup BEFORE
migrations run, so a botched future upgrade is recoverable by restoring
a single file instead of a data-loss incident.
Uses SQLite's VACUUM INTO — safe under WAL, cannot tear against
concurrent writes. Best-effort: failures are logged, never raised —
the main DB remains the source of truth.
Configurable via NOTIFY_BRIDGE_PRE_MIGRATE_SNAPSHOT_KEEP (default 5;
0 disables). Snapshots land in ``data_dir/backups/pre-migrate-<ts>.db``
and the N oldest are pruned each boot.
