fix(docker): move dedup helper to a model-free module (CI green)

The widget-dedup test imported plugins.docker.routes, which re-imports
plugins.docker.models after the app already loaded the docker plugin —
"Table 'docker_containers' is already defined". Same plugin-loader gotcha the
host_agent query helpers avoid.

Extract dedup_by_container_id into plugins/docker/dedup.py (imports no ORM
models, so it's safe to import in either environment); routes.py imports it
from there and the test targets the model-free module.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016Jg27rgypiW2efULXJDtMC

tests/integration/test_docker.py

@@ -456,14 +456,14 @@ def test_widget_dedup_collapses_cross_manager_duplicates():
     the dashboard widget must count it once. Older agents send no container_id,
     so those rows can't be matched and are all kept."""
     from types import SimpleNamespace
-    from plugins.docker.routes import _dedup_by_container_id
+    from plugins.docker.dedup import dedup_by_container_id
 
     def c(cid, name, status="running"):
         return SimpleNamespace(container_id=cid, name=name, status=status)
 
     rows = [c("abc", "web@m1"), c("abc", "web@m2"),  # same task, two managers
             c("def", "db"), c("", "noid-1"), c("", "noid-2")]
-    out = _dedup_by_container_id(rows)
+    out = dedup_by_container_id(rows)
 
     assert len(out) == 4                                       # one "abc" dropped
     abc = [r for r in out if r.container_id == "abc"]