Three related fixes around 4K-source transcoding that left the web
player stuck on "preparing session" with no useful diagnostics:
1. Dynamic -level:v derived from output height (hls.go, transcoder.go).
The previous fixed "4.0" silently rejected anything taller than 1080p
inside libx264 — "frame MB size > level limit", "DPB size > level
limit" — and emitted unplayable segments. Helper H264LevelForHeight()
now picks 4.0 / 5.0 / 5.1 / 6.0 from the actual encode height.
2. New `unarr probe-hwaccel` diagnostic command. Lists the HW encoders
compiled into ffmpeg, the device files / drivers present, and the
backend the daemon would actually pick today. Surfaces the canonical
gotcha: a host with an RTX 3090 + nvidia-smi but a Homebrew ffmpeg
built without --enable-nvenc still falls back to libx264 software.
3. Register payload now includes hwAccel + maxTranscodeHeight so the web
side can suggest a smaller alternate quality before the user even
tries to play a 4K source on a software-only host. Software-only =
1080p cap, any HW backend = 2160p cap.
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