skydick: fix qbittorrent UID collision with ylw ...

UID 1002 was already assigned to ylw on skydick. Change qbittorrent
system user to UID 900 to avoid the collision. NFS sec=sys maps by
UID number, so this must not conflict with any normal user.

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

skydick: add qbittorrent user and make media NFS export writable ...

Add qbittorrent system user (UID 1002, group storage) for NFS
root_squash write access. Change /srv/media export from ro,async
to rw,sync to support *arr torrent downloads under /srv/media/torrents/.

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

skydick: add NVMe SLOG+L2ARC documentation for datapool ...

Intel DC P4600 750GB: 8GB SLOG partition for sync write acceleration,
remaining ~690GB as L2ARC read cache for VM working sets.

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

skydick: fill in actual Mach2 WWNs for datapool creation ...

5x ST14000NM0001 14TB SAS Mach2 drives, 4 active + 1 spare.
Mirror vdevs pair LUN0/LUN1 from different physical drives.

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

skydick: extract datapool.nix for Mach2 ZFS storage config ...

Move all storage-serving config (NFS, Samba, iSCSI, tmpfiles, firewall
ports, storage group) from default.nix into datapool.nix. Add Mach2
dual-actuator mirror layout documentation, new datasets (torrent, vm),
and clean permission model (setgid storage group for user data).

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