4K transcodes and HDR tone mapping

A 2160p level (H.264 ~25 Mbit/s) joins the ladder, used when the server allows it (Transcoding:MaxHeight 2160, MaxVideoKbps ~30000; a hardware encoder). libx264's level and the master playlist's CODECS follow the height (5.1 / avc1.640033 for 4K, 4.1 below). The quality menu lists only the levels a file can have (AdaptiveLevels: the server's maximum, the user's cap, the picture), not every level.

HDR video (PQ or HLG, from ffprobe's color_transfer) that's transcoded is tone-mapped to SDR BT.709: scaled down, then zscale to linear light from the probe's transfer and BT.2020, Hable tonemap, zscale to BT.709, tagged BT.709; on the CPU before any hardware upload. On when ffmpeg has zscale and tonemap (checked at start, logged if not) and Transcoding:ToneMapHdr (default true). Docs, changelog, tests.

Co-Authored-By: Claude Opus 5.5 noreply@anthropic.com