Land a file the browser has sent to storage.

Verifies the staged object (it exists and fits the type's ceiling), then persists it like POST /files/upload. A file within the buffered ceiling is read back and analyzed in the request, so the response is the same (state='pending' with sheets when the import wizard is needed, or the dataset stubs being parsed). A larger CSV or Parquet file is committed and parsed by the worker, which creates its datasets (databases is empty). Completing an upload that already landed returns that file.