feat: decode and re-encode video on the GPU where there is one

A 1080p film is decoded by whoever watches it and re-encoded by the node
when no browser can decode the source. Both were on the CPU, and on an
Atom or Celeron mini-PC neither reaches real time — which is what
`transcode_incompatible_video` exists to refuse. This adds the mechanism
that makes refusing it unnecessary.

Node — `hwaccel.py`: VA-API on Linux, Quick Sync or NVENC on Windows,
established by encoding 1080p and reading the file back with ffprobe.
Nothing is accepted that does not produce the exact profile and level
`stream_init` announces, since the client checks that string before it
trusts a byte: an encoder that wrote another level would make the node's
own codec string a lie, and ffmpeg takes `-level 4.1` and `-level 41`
from h264_qsv without saying which it understood. Three modes per
stream — hardware decode and encode, hardware encode alone, libx264 —
demoted per source codec, because a GPU that decodes HEVC may have no
decoder for MPEG-4 Part 2 and only asking it finds out. A mode that
fails is detected on an empty stdout before `stream_init` goes out, so
the viewer sees one working stream and never an error.

Client — Chromium ships VA-API off on Linux. It is enabled where a
render node and a driver are present, then verified through
`navigator.mediaCapabilities`: a no moves to the next GL backend on the
next launch and an exhausted list drops the switches, so a renamed
feature cannot pass for a feature that is on and `--ignore-gpu-blocklist`
cannot survive on a machine it did not help. Feature lists now merge
rather than overwrite — `appendSwitch` replaces the value, and a second
caller would have silently cancelled the mDNS switch aiortc depends on.

Packaging — the drivers are weak dependencies on all four formats, so a
machine without a GPU installs exactly as before. `dpkg -i` and
`rpm -ivh` ignore weak deps; `packaging/README.md` now says so. Windows
needs no driver: the bundled ffmpeg already carries h264_qsv and
h264_nvenc, and a re-pin that dropped them would cost every low-power
Windows node its hardware encoding silently.

AMD on Windows (AMF) and macOS (VideoToolbox) are named gaps, not
oversights: neither could be tried anywhere in this project, and both
re-encode in software as before.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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