feat(transcode): dynamic H.264 level + HW probe + capability reporting

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.

internal/cmd/root.go

@@ -104,6 +104,8 @@ Source:         https://github.com/torrentclaw/unarr`,
 	statsCmd.GroupID = "system"
 	doctorCmd := newDoctorCmd()
 	doctorCmd.GroupID = "system"
+	probeHWAccelCmd := newProbeHWAccelCmd()
+	probeHWAccelCmd.GroupID = "system"
 	cleanCmd := newCleanCmd()
 	cleanCmd.GroupID = "system"
 	selfUpdateCmd := newSelfUpdateCmd()
@@ -140,6 +142,7 @@ Source:         https://github.com/torrentclaw/unarr`,
 		// System & Diagnostics
 		statsCmd,
 		doctorCmd,
+		probeHWAccelCmd,
 		cleanCmd,
 		selfUpdateCmd,
 		versionCmd,