Move User File Endpoint

Move a file into a folder (or to the root if target_folder_id is None).

Side effects when source/target folders are merge-enabled:

• source-side: drop the file's slice from the source collection (parquet rewrite + DatasetMember delete) so dashboards reflect the move immediately.
• target-side: re-arm the file's databases and dispatch a Temporal parse so the rows land in the target collection.

Filename collisions in the destination are auto-resolved with foo (1).csv style suffixes.