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>
bvandeusen