Created engine/tasks.py with:
- execute_run and execute_sweep Celery tasks registered via autodiscover
- SyncTaskResult class mimicking Celery AsyncResult for in-process mode
- dispatch_run/dispatch_sweep helpers that route to Celery or sync based on config
- Proper async-to-sync bridging for the async engine functions
- 17 tests covering task execution, sync fallback, error handling, and Celery dispatch
