The shared AsyncPostgresSaver caches DB connections in a module-level
pool. Cached connections are bound to the asyncio loop that opened
them, but `run_async_celery_task` discards the loop on each task's
exit — so after the first task the pool holds connections pointing
to a dead loop, and the next automation hangs 30s before failing
with `PoolTimeout: couldn't get a connection after 30.00 sec`.
Swap agent_task to `InMemorySaver`; automation runs only need state
within one Celery task, so nothing is lost. Site-local TODO tracks
the proper future fix (dispose the checkpointer pool around each
Celery task, mirroring `_dispose_shared_db_engine`).
