Asking for 2160p quality on a 720p source kept the daemon's qcap.VideoBitrate
at 25 Mbps even after outputHeight was clamped to the source. The level
H264LevelForHeight picks for the 720p output is 3.1 / 4.0, which rejects any
VBV >20 Mbps — libx264 then exited with "VBV bitrate (25000) > level limit"
on every restart, ffmpeg auto-restarted 3 times, master.m3u8 never appeared,
and the player got stuck at "Preparando sesión".
Re-derive the (height, bitrate) cap from the EFFECTIVE outputHeight via the
new capForHeight helper. Result: 720p source asked for 2160p → outputs 720p
with the 3500 kbps bitrate the level actually accepts. ffmpeg runs cleanly,
master.m3u8 appears, playback starts.
The web also clamps effectiveQuality to source resolution before the session
row is written, so the daemon mostly receives sane labels. This change keeps
the daemon defensive against (a) older web clients that still ask for
upscaled qualities, and (b) future quality="original" requests where qcap
is empty and Transcode.VideoBitrate could overshoot the level too.
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