POST /deployments/impact returns 500 with unhandled psycopg IntegrityError when a node in the spec already exists

[shangyian]

POST /deployments/impact returns HTTP 500 with an unhandled psycopg.errors.UniqueViolation on unique_node_namespace_name when the deployment spec contains a node that already exists. Impact analysis should reconcile an existing node as an update (or report it in the impact result), and a genuine conflict should surface as a typed DJAlreadyExistsException (409), not a raw DB error escaping as a 500.

Repro (v0.0.193):

1. Create transform demo.scratch.foo.
2. POST /deployments/impact with a spec that bulk-creates demo.scratch.foo plus another node.
3. Server bulk-INSERTs into node, hits unique_node_namespace_name, and the IntegrityError propagates unhandled → 500.

Source: node-create path in datajunction_server/internal/deployment/orchestrator.py. A related unhandled TypeError occurs at datajunction_server/internal/deployment/orchestrator.py:1669 in _deploy_links on the same endpoint.

Fix: treat already-existing nodes as updates during impact/deploy, wrap node-create so a uniqueness conflict raises DJAlreadyExistsException (409) instead of leaking psycopg.IntegrityError, and guard the _deploy_links type error.