skydick: redesign datapool with per-user datasets and service model ...

Replace flat purpose-first layout (share/media/torrent/backup/vm) with
user-first hierarchy:

- dick/public: shared collaborative files
- dick/media: shared media with data/ + library/ in one hardlink domain
- dick/users/<user>/{files,bt-state,vm}: per-user private trees with
  ZFS quotas, per-user NFS all_squash, and Samba [homes]
- dick/system/{backup,vm}: admin-only system datasets
- dick/templates/vm: read-only shared VM base images

NFS exports split media into rw writer (all_squash to qbittorrent) and
ro reader (/media/library).  Per-user exports use explicit anonuid/gid.
Samba uses [public] for shared, [homes] for per-user, [media] ro for
library.  Legacy exports preserved for active migration.

Add DATAPOOL.md with user/admin guide covering SMB/NFS connection,
new-user provisioning, quotas, and troubleshooting.

Co-Authored-By: Claude Opus 4.6 <[email protected]>