doc: FTTH migration — gateway is now 192.168.0.1

The Archer C7 at .2 is gone; the FTTH box at .1 is the gateway. Every
static holdout (nas host, nuc host, privoxy, transmission-bt) pointed at
the dead .2 and had no internet — the reported symptom was privoxy.

Also record two things the migration broke that were not obvious:

- DHCP reservations did not carry over. blocky held .254 by reservation
  on the C7; a lease renew on the FTTH box moved it and took LAN DNS
  down. It is static now. jellyfin-* are still DHCP on stale leases.
- The FTTH box advertises native IPv6. transmission-bt's tunnel is
  AllowedIPs = 0.0.0.0/0, so v6 egressed around the kill switch on the
  home address. IPv6 is now disabled in that container.

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

nas/transmission-bt.md

@@ -66,17 +66,17 @@ network:
         addresses: [192.168.0.254]
       routes:
         - to: 193.70.35.17/32
-          via: 192.168.0.2
+          via: 192.168.0.1
 EOF
 chmod 600 /etc/netplan/10-lxc.yaml
 netplan apply'
 
 # packages need a temporary default route (removed right after)
-incus exec "$CNAME" -- ip route add default via 192.168.0.2
+incus exec "$CNAME" -- ip route add default via 192.168.0.1
 incus exec "$CNAME" -- apt-get update
 incus exec "$CNAME" -- apt-get install -y --no-install-recommends \
     transmission-daemon wireguard-tools iptables curl
-incus exec "$CNAME" -- ip route del default via 192.168.0.2
+incus exec "$CNAME" -- ip route del default via 192.168.0.1
 
 # WireGuard full tunnel (generate key, print pubkey for the ks4 side)
 incus exec "$CNAME" -- bash -c 'umask 077