POST /topologies/{id}/lans previously called _auto_attach_gateway()
whenever a non-DMZ LAN was created, which wired the DMZ gateway decky
to every new subnet. That's why a deployed gateway ended up with
eth0..ethN on every LAN regardless of what the user drew in MazeNET.
Drop the auto-attach helper entirely. The DMZ_ORPHAN deploy-time
validator (decnet/topology/validate.py:65-110) stays strict — users
must explicitly wire the gateway to each subnet they want bridged,
which is the whole point of having a topology editor.
useMazeApi.ts: drop stale auto-bridge reference from comment.
When a non-DMZ LAN is created via POST /lans, look up the topology's
gateway (decky with forwards_l3=True attached to the DMZ) and insert
an edge binding it to the new LAN. The gateway becomes multi-homed
to every internal LAN automatically, so DMZ_ORPHAN cannot arise
from ordinary editor use.
Also fixes delete_lan: the home-decky guard used scalar_one_or_none,
which blew up when the gateway already had >1 'other' LAN edge.
Switch to scalars().first() — we only need to know *some* other
edge exists, not a unique one.
