fix(node,hub): always transcode video audio to stereo AAC, never copy

MSE only decodes AAC/Opus, so copying a source's real audio codec left
non-AAC files silently unplayable in-browser (E-AC-3 additionally made
ffmpeg itself refuse to write the fragmented MP4 header). Audio is now
always transcoded to AAC and downmixed to stereo — multichannel AAC is
accepted by ffprobe/VLC but silently rejected by some browsers' MSE
decoder once real fragments are appended, which forces the SourceBuffer
out of its MediaSource with no explicit error. Video stays copy-only.

Also: report a clear client-side error instead of a bare STREAM_END when
ffmpeg exits nonzero before producing any output, add video-element/
MediaSource error logging on the client for the next time this class of
bug needs diagnosing, and fix a hub test that had grown too broad a scan
window after an earlier, unrelated transport.js change.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016SF6RKNBKg9qejmoMJ9ybA

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