Adds WebRTCSession.Quality to the sync payload so the daemon can pick a
MaxHeight + bitrate per session instead of using the global config cap.
resolveQualityCap() maps the label to a (height, b:v) pair and
buildStreamSource() promotes a passthrough decision to ActionTranscodeVideo
when the source resolution exceeds the cap (4K source on a phone client
with quality="720p" must transcode, not pass-through).
Also lands the transcode-on-by-default fix for legacy configs without a
[downloads.transcode] section so existing installs pick up h264+aac
fallback for HEVC/AC3 content without re-running setup.
70f7337226