Move backup to /volume2/NetBackup (DSM shared folder, kts_bak read/write) and
change the design from a --delete mirror to an additive, date-stamped archive:
per-run pg_dump snapshots for the DBs and per-run full tar.gz snapshots for
audio, each kept for 7 generations, transferred over encrypted SSH with
post-transfer SHA256 verification (success only when byte-identical). This
removes the --delete risk (source-side deletion no longer wipes the backup),
enables restore-to-a-date, and is verified by an actual restore from the NAS
into isolated DBs (0 errors, quota/ledger and audio associations consistent).