/upload/complete was failing (non-JSON response, missing fileId, or
post-processing timeout) after all bytes were already on the server,
causing upload-manager to retry the entire multi-GB upload — which
corrupts the server-side file since two uploads end up interleaved.
Now /complete failures are swallowed and sessionId is used as the
file_code fallback. Upload is considered done once all chunks are in.
Upload completes on server but file is still being processed, so
share-link fails. Retry up to 6x with backoff; on final failure, use
fileId-based fallback URL instead of throwing — prevents upload-manager
from retrying the entire multi-GB upload.