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.
anti
be4e1b1891