Adds the `target_host_uuid` FK on `Topology` plus wiring through the
two create endpoints (`POST /topologies`, `POST /topologies/blank`).
Validates the mode/host pair: `mode='agent'` now requires a known,
routable host; `mode='unihost'` must leave the field unset.
Surfaced on `TopologySummary` so list/detail responses expose it.
Purely additive at the schema level — existing unihost flows unchanged
(field defaults to `NULL`).
Step 1 of the agent <-> topology integration.
POST /topologies/blank seeded the gateway decky with
archetype=host-gateway + network_mode=host, but neither was wired:
no compose writer reads network_mode and host-gateway is not a real
archetype. Replace with archetype=deaddeck + forwards_l3=true so the
gateway is a normal multi-homed bridge decky, consistent with how
compose.py interprets forwards_l3 (sysctl + NET_ADMIN).
Edge marked is_bridge=true, forwards_l3=true so downstream readers
(generator, compose, validator) see a real bridge attachment.
