refactor(fc2c-i): sweep blueprints + ml tasks onto shared get_session (covers tasks/ml.py, a survey gap)

backend/app/tasks/ml.py

@@ -305,13 +305,11 @@ def _confidence_for_tag(session, tag, preds: dict) -> float | None:
 def recompute_centroid(self, tag_id: int) -> bool:
     import asyncio
 
-    from ..extensions import make_engine, make_session_factory
+    from ..extensions import get_session
     from ..services.ml.centroids import CentroidService
 
     async def _run() -> bool:
-        engine = make_engine()
-        Session = make_session_factory(engine)
-        async with Session() as session:
+        async with get_session() as session:
             svc = CentroidService(session)
             result = await svc.recompute_for_tag(tag_id)
             await session.commit()
@@ -325,13 +323,11 @@ def recompute_centroids(self) -> int:
     """Daily: find drifted centroids, enqueue recompute_centroid for each."""
     import asyncio
 
-    from ..extensions import make_engine, make_session_factory
+    from ..extensions import get_session
     from ..services.ml.centroids import CentroidService
 
     async def _list() -> list[int]:
-        engine = make_engine()
-        Session = make_session_factory(engine)
-        async with Session() as session:
+        async with get_session() as session:
             return await CentroidService(session).list_drifted()
 
     drifted = asyncio.run(_list())