Add per-workload capture of host-bind data volumes as downloadable tar.gz archives: a new internal/volsnap engine (enumerate host-bind volumes via the computeMounts merge, archive with archive/tar+gzip skipping symlinks/special files, per-workload retention + startup orphan cleanup), a volume_snapshots table + store CRUD, admin-gated API (list/snapshotable/create/download/delete), and a Snapshots panel on /apps/[id] that shows coverage and which volumes are skipped (and why). Scope: image-source apps, host-bind scopes (absolute/stage/project); Docker named volumes, tmpfs, and instance scope are surfaced as not-yet-supported. Restore is a separate later phase. Download/FilePath are containment-checked; create returns a typed no-data error (400) vs generic 500. Covered by archiver unit tests + full API e2e.
