Tests: stop the ``QueueProcessorV2`` background thread between tests in the ``reset_singletons`` autouse fixture (it already did this for ``BackgroundJobScheduler``). The module-level ``queue_processor`` singleton was started by the first ``create_app()`` in an xdist worker and never stopped, so one test's processor thread ran for the whole worker — looping through ``SettingsManager`` → SQLCipher connection opens on the shared ``db_manager`` concurrently with every later test, and emitting logs to a closed pytest stderr sink at teardown (the same class of bug the scheduler handling fixes). Tests that exercise the queue patch the singleton, so stopping the real thread does not affect them.
