doc: build the ks4 -> nas backup pull leg

FTTH is up, so the leg nas-seed.md had been holding since 2026-08-30 is
now built: wg-ks4 on nas (10.8.0.22), nas peered on ks4's wireguard
container, incus remote over the tunnel, 05:00 cron, and the first pass
seeding under a systemd-run unit.

Three corrections the runbook needed, all found by running it:

- nas had no wireguard-tools at all. transmission-bt carries its own
  tunnel inside the container, so the host never needed them.
- Every ks4 instance has an instance-level eth0 pinned to incusbr0 with
  a static 192.168.1.x, so each copy failed in under a second with
  "Cannot use manually specified ipv4.address when using unmanaged
  parent bridge". nas now runs a managed incusbr0 on 192.168.1.254/24 —
  deliberately not .1, which must keep resolving over wg-ks4.
- The seed command was missing -p backup, which the doc's own
  verification step already assumed.

Also records the measured rate: ~125 Mbit/s, ks4's OVH uplink rather
than the home downlink, so ~31 h for the first pass — during which the
shared incus-copy lock suppresses the 04:00 nasbackup job.

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

nas/README.md

@@ -36,12 +36,26 @@ Storage + backup host on the LAN, added 2026-08.
 | privoxy | 192.168.0.11 | — | unprivileged, autostart; filtering HTTP proxy, listens on **:3128** (not privoxy's default 8118); static config in `/etc/systemd/network/eth0.network` (`Gateway=192.168.0.1`), `DNS=192.168.0.254`. Moved from nuc 2026-08-30 |
 | [transmission-bt](transmission-bt.md) | 192.168.0.7 | ✅ | unprivileged, autostart; always-on WireGuard full tunnel → ks4 (egress = 193.70.35.17, kill switch: no default route in `main`, wg-quick's `fwmark`/`suppress_prefixlength` rules send traffic to table 51820 — **`netplan apply` wipes those rules, so always `systemctl restart wg-quick@wg0` after it**); **IPv6 disabled** (`/etc/sysctl.d/99-no-ipv6.conf`) since the tunnel is `AllowedIPs = 0.0.0.0/0` only and the FTTH box's native IPv6 RA bypassed the kill switch entirely. Extending the tunnel to `::/0` is **not currently possible**: ks4 has a global v6 address and a default v6 route but **no working v6 egress** (verified 2026-09-16 — both ICMP and TCP to the v6 internet fail while v4 is fine), so it cannot act as a v6 exit. Fix OVH v6 on ks4 first if v6 peers are ever wanted; `/export/media` disk device (`shift=true`), downloads to `/media/downloads`; web UI :9091 (LAN only). Moved from nuc 2026-08-30 |
 
+## Host tunnel
+
+`wg-ks4` — `10.8.0.22/24`, peer = the `wireguard` container on ks4,
+endpoint `193.70.35.17:51845`, `AllowedIPs = 10.8.0.0/24, 192.168.1.1/32`.
+It exists only to reach ks4's incus API at `192.168.1.1:8443` for the
+05:00 pull. Key at `/etc/wireguard/wg-ks4.key`, unit
+`wg-quick@wg-ks4` (enabled).
+
+⚠️ nas also runs a **managed `incusbr0` on `192.168.1.254/24`** — no
+uplink, nothing attached, inert. It exists purely so the ks4 replicas'
+instance-level `eth0` (`parent: incusbr0`, static `192.168.1.x`) passes
+validation on arrival. It must never take `192.168.1.1`: that address has
+to keep resolving over `wg-ks4`, and a local address beats a route.
+
 ## Backup pools hosted here
 
 | incus pool | dataset | receives |
 |---|---|---|
 | `nucbackup` | `tank/backup/nuc` | nuc's instances (pushed nightly, 03:30) |
-| `ks4backup` | `tank/backup/ks4` | ks4's instances (pulled over WG, 05:00 — after FTTH) |
+| `ks4backup` | `tank/backup/ks4` | ks4's instances (pulled over `wg-ks4`, 05:00; built 2026-09-16, see [ks2/nas-seed.md](../ks2/nas-seed.md)) |
 | `nasbackup` | `tank/backup/nas` | **nas's own** instances (local copy, 04:00) |
 
 nas's own instances are replicated **locally** rather than to nuc: nuc is

nas/nas-install.md

@@ -710,11 +710,15 @@ is now here and carries its own).
   remove `/etc/wireguard/wg-ks4.conf`, and drop the `10.8.0.20/32` peer
   on ks4.
 
-Seed manually in tmux — the first pass is **~1.75 TiB** over the home
-downlink and only makes sense once FTTH is up:
+Built 2026-09-16 — full runbook, corrections and gotchas:
+[ks2/nas-seed.md](../ks2/nas-seed.md). Two things that block the copy if
+missed: nas needs `wireguard-tools` installed and a **managed `incusbr0`
+on `192.168.1.254/24`** (never `.1`), and the seed needs **`-p backup`**.
 
 ```sh
-/root/scripts/incus-copy.sh -r ks4 -s ks4backup 2>&1 | tee -a /var/log/incus-copy-ks4.log
+systemd-run --unit=ks4-seed --collect \
+  /bin/bash -c '/root/scripts/incus-copy.sh -r ks4 -s ks4backup -p backup \
+                >> /var/log/incus-copy-ks4.log 2>&1'
 ```
 
 Then test-restore one instance before ticking the gate in