FileMode.Append forces all writes to the end of the file, making
fs.Position assignments ineffective. This works by accident on a
fresh sequential upload but will silently corrupt the file on any
retry, since chunks are re-sent from their original offset but land
at the current end instead. Use FileMode.OpenOrCreate so the seek
to chunk.Start is honoured.
/api/Upload/Init and /api/Upload/Chunk had no authorization policy,
meaning any unauthenticated request could create archive database
records and write arbitrary data to server storage. Every other
endpoint group in the application requires authorization; apply
the same administrator role requirement here.
