fix(ml): per-task async engine for recompute_centroid (#881)

recompute_centroid + recompute_centroids were the only tasks still using
the process-wide singleton extensions.get_session() under asyncio.run().
The async engine's asyncpg pool is bound to the loop it was created on;
each Celery task runs a fresh asyncio.run() loop, so after the first
invocation the cached engine handed loop-A connections to loop B and raised
"Future attached to a different loop" — every recompute after the first in
a worker process failed (~35ms, fails on first DB await).

Convert both to the established per-task async_session_factory() pattern
(NullPool engine created + disposed inside the task's own loop), matching
scan/download/admin tasks. No get_session usages remain in tasks/.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/tasks/ml.py

@@ -491,15 +491,22 @@ def _confidence_for_tag(session, tag, preds: dict) -> float | None:
 def recompute_centroid(self, tag_id: int) -> bool:
     import asyncio
 
-    from ..extensions import get_session
     from ..services.ml.centroids import CentroidService
+    from ._async_session import async_session_factory
 
     async def _run() -> bool:
-        async with get_session() as session:
-            svc = CentroidService(session)
-            result = await svc.recompute_for_tag(tag_id)
-            await session.commit()
-            return result
+        # Per-task NullPool engine bound to THIS asyncio.run loop — the shared
+        # process-wide engine reuses connections across loops and raises
+        # "Future attached to a different loop" on every call after the first.
+        async_factory, async_engine = async_session_factory()
+        try:
+            async with async_factory() as session:
+                svc = CentroidService(session)
+                result = await svc.recompute_for_tag(tag_id)
+                await session.commit()
+                return result
+        finally:
+            await async_engine.dispose()
 
     return asyncio.run(_run())
 
@@ -515,12 +522,17 @@ def recompute_centroids(self) -> int:
     """Daily: find drifted centroids, enqueue recompute_centroid for each."""
     import asyncio
 
-    from ..extensions import get_session
     from ..services.ml.centroids import CentroidService
+    from ._async_session import async_session_factory
 
     async def _list() -> list[int]:
-        async with get_session() as session:
-            return await CentroidService(session).list_drifted()
+        # Per-task NullPool engine bound to this loop (see recompute_centroid).
+        async_factory, async_engine = async_session_factory()
+        try:
+            async with async_factory() as session:
+                return await CentroidService(session).list_drifted()
+        finally:
+            await async_engine.dispose()
 
     drifted = asyncio.run(_list())
     for tid in drifted: