feat(transcode): tonemap HDR sources to SDR (zscale-gated)

HDR (HDR10/HLG/Dolby Vision) transcoded to SDR came out washed-out and
desaturated because the filter chain never tonemapped. buildHLSFFmpegArgsAt now
inserts a zscale linearise -> hable tonemap -> BT.709 chain after the scale and
before format=, but only when the source is HDR and the ffmpeg build has zscale
(FFmpegSupportsZscale, cached). Builds without zimg keep the old behaviour
(plays, just desaturated) instead of erroring.

It's a CPU filter, valid for every encoder here: the decode hwaccel deliberately
leaves frames in system memory (no -hwaccel_output_format), so zscale runs ahead
of format=/hwupload exactly like the existing scale filter. Verified on a real
4K HDR10 file — vivid colour and deep blacks vs the washed-out baseline.

internal/engine/transcode_quality.go

@@ -14,6 +14,10 @@ type TranscodeRuntime struct {
 	// browser-friendly. Useful when the user explicitly turns transcoding
 	// off in config.
 	Disabled bool
+	// TonemapHDR enables HDR→SDR tonemapping of HDR sources during transcode.
+	// Set only when the ffmpeg build has zscale (FFmpegSupportsZscale); without
+	// it the tonemap filter would error and break playback, so it stays off.
+	TonemapHDR bool
 }
 
 // qualityCap maps a session's Quality label to a (MaxHeight, VideoBitrate)