fix(backend): expose c2_task.source in GET /c2/tasks response

Serialize source as t.source.value (string) in list_simulation_tasks.
Updated test_c2_tasks_list shape assertion to include 'source' and
assert value is 'mimic' for execute-created tasks. Added test in
test_c2_import to assert source='import' in GET /c2/tasks after import.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/api/c2.py

@@ -360,6 +360,7 @@ def list_simulation_tasks(sid: int):
                 "status": t.status,
                 "completed": t.completed,
                 "output": t.output,
+                "source": t.source.value,
                 "mapping_applied": t.mapping_applied,
                 "created_at": t.created_at.isoformat() if t.created_at else None,
                 "completed_at": t.completed_at.isoformat() if t.completed_at else None,