Finalize an upload session created by Initiate single-PUT upload (POST /v2/files/upload). The server HEADs the S3 object to confirm bytes are present and that the size matches the declared value, then flips the file row to DONE.

On success the file is marked as uploaded and returned in the response. The file's processing status may still be pending while downstream processing runs.

Idempotent for already-DONE files: a retry of a previously-successful complete call (or a complete call arriving after the auto-finalize backstop has already finalised the row) returns 200 with the file metadata instead of 409. FAILED / CANCELLED still return 409.

To cancel an in-progress upload, simply stop calling the API: abandoned file records are garbage-collected after 24 hours.