Add GET /thumbnail to the agent stream server: ffmpeg extracts one frame
at a timestamp (-ss before -i, single-frame MJPEG to stdout) for the web's
file-characteristics panel. Auth via a token scoped thumb:<sha256(path)>
(same HMAC scheme as /stream and /hls; the web mints, the agent verifies),
clamped to a real regular file, 404-no-oracle on a bad token, 20s timeout.
ffmpeg path wired into the stream server from the daemon. Version -> 0.13.0.
