nas: new storage host on A1SAi-2750F — 4 TB off USB onto SATA

Builds the box that ends the usb4t dropouts: the JMicron bridge was the
least reliable device in the setup and it held the intended off-site copy
of ks4. The 4 TB now sits on direct SATA as pool `tank`.

- OS on mdraid RAID1 across two 120 GB SSDs (Intel 330 + Toshiba Q300),
  both ESPs bootable; `incus` ZFS mirror on their tails, ~22% left
  unallocated as over-provisioning
- media at /export/media, exported read-only over NFSv4 to nuc
- transmission-bt moves here (its WireGuard tunnel is in-container, so
  ks4 needed no change) and writes to the dataset locally
- backup pools nucbackup / ks4backup / nasbackup
- monitoring live: msmtp (submission+auth, verified 250), zed with
  NOTIFY_DATA, zpool-health.sh every 15 min, smartd on all three disks

Traps recorded because none of them point at their own cause: booting
with the display active kills the i915 probe and wedges incus; d-i picks
grub-pc vs grub-efi from how the installer booted; `incus storage create`
hangs forever on a mountpoint=none dataset; the BMC is deliberately never
cabled, so there is no out-of-band console.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -12,7 +12,9 @@ backed up, by which tool, on what schedule, and how to restore.
 service to ks4 (and getting it backed up automatically).
 
 **Tech notes**
-- [`nuc/`](nuc/README.md) — home lab on `nuc`: host, storage, instances
+- [`nuc/`](nuc/README.md) — home lab on `nuc`: host, iGPU instances
+- [`nas/`](nas/README.md) — storage + backup host `nas` (Supermicro
+  A1SAi-2750F): the 4 TB on direct SATA, media over NFS, backup pools
 - [`ks4/`](ks4/README.md) — prod server `ks4` at OVH: host, services, network flows
 - [`ks2/`](ks2/) — legacy backup server being [decommissioned](ks2/plan.md)
 - [`archer-c7/`](archer-c7/) — home router (TP-Link Archer C7 v5 running on OpenWrt)