The CE safety commit (8a3485454a) converted module-level dicts to lazy
functions but forgot to update __init__.py, which still imported the
now-deleted TRACE_TASK_TO_CASE constant causing an ImportError at startup.
Add get_trace_task_to_case() to gateway.py as a lazy public wrapper
(inverse of _get_case_to_trace_task) and update __init__.py to call it.