Daemon now runs engine.DetectHWAccelDiagnostic at startup (instead of the
lighter DetectHWAccel) and ships the full picture — ffmpeg version,
resolved binary path, HW encoders compiled in, device files / drivers
detected — up to the server in the RegisterRequest payload.
Why: the most common cause of slow first-play is a software-only ffmpeg
build. Surfacing the diagnostic in the web AgentsTab "Diagnose
transcoder" modal lets a user see *why* their backend landed on libx264
(e.g. brew's default formula ships without --enable-nvenc, or the
container is missing /dev/nvidia0) without SSHing in to run `unarr
probe-hwaccel` manually.
Also emits a single `[transcode]` startup log line summarising the same
data — convenient for `journalctl --user -u unarr | grep transcode`.
Bounded by a 10 s context so a hung ffmpeg binary can't stall daemon
startup forever.
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