Remove duplicate folder scanning feature

Delete folder_rename_service.py. Stub out get_duplicate_folders API to return
empty response. Update folder_scan_service and tests to skip rename step.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/server/services/scheduler/__init__.py

@@ -19,6 +19,4 @@ __all__ = [
     "SchedulerServiceError",
     "get_scheduler_service",
     "reset_scheduler_service",
-    # Sub-services (still in scheduler folder)
-    "folder_rename_service",
 ]