Address round-1 review on #913 and the SonarCloud new_reliability_rating gate:
- credential_cleanup_task no longer catches the cancellation exception
(Sonar python:S7497). A task-group cancel must propagate for structured-
concurrency teardown; graceful shutdown still flows through shutdown_event,
so the sleep no longer needs a cancel/break.
- Parametrize the scanner self-heal tests over 401 AND 403 (handled
identically at both call sites) and add a test that a failing periodic
sweep is logged non-fatally and does not crash the task.
- Log the stored-user count before the startup sweep (operability signal),
add a debug line when the credential row was already gone, and document
the at-most-one extra-401 convergence in _remove_stale_credential.
Refs Deck #198.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
