POST /topologies raised a 500 with a raw SQLAlchemy IntegrityError
traceback when the name collided with an existing topology. Catch
the error at the router, verify it's the ix_topologies_name
constraint (so unrelated integrity failures still surface as 500s
with their real traceback), and return 409 with a helpful detail.
Test covers the create-then-duplicate-create flow.
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.
