Stream-request failures previously reported status:"failed", which the web
treated as a download failure — it left the task unstreamable and surfaced a
misleading 20s timeout. Report them through a dedicated StreamError field
instead, so the web clears the stream flag and shows the real reason without
touching the download status.
- StatusUpdate gains StreamError (json: streamError)
- OnStreamRequested reports failures via a reportStreamError helper (path
rejected, file not found, no video in dir) instead of status:"failed"
- os.Stat is retried 3× (300ms) before giving up — NFS can transiently fail
(ESTALE/EAGAIN/timeout), the root of the intermittent "works on the 3rd try"
- dispatch OnStreamRequested off the sync loop (goroutine): it does blocking
I/O (stat retries, ffprobe in SetFile) that would otherwise stall task
dispatch + status reporting for other items