feat(plex): S1a — add intel-media-va-driver for VAAPI transcode (E6)

The runtime image ships ffmpeg with h264_vaapi/libva2 but no VA driver, so
h264_vaapi can't initialise and a Plex transcode silently falls back to
libx264. Add the iHD driver (intel-media-va-driver) so the UHD 630 iGPU can
hardware-encode once /dev/dri is passed into LXC 118 (host + compose side,
tracked in siftlode-ops). Harmless where no render node is present.

Dockerfile

@@ -45,12 +45,15 @@ RUN adduser --disabled-password --gecos "" appuser \
     && mkdir -p /downloads \
     && chown appuser /app /downloads
 
-# ffmpeg: merge separate video+audio streams + postprocessing. Deno: the JavaScript runtime
-# yt-dlp uses to solve YouTube's "n" signature challenge for the web player clients (required
-# alongside the PO token, else only storyboards are offered). yt-dlp auto-detects deno on PATH.
+# ffmpeg: merge separate video+audio streams + postprocessing, and the Plex VAAPI transcode (E6 S1).
+# intel-media-va-driver (iHD): the VA driver ffmpeg's h264_vaapi needs — libva2 ships in the base but
+# no driver, so without this `vainfo`/`h264_vaapi` fail and transcode silently falls back to libx264.
+# Harmless on hosts with no /dev/dri (the driver just goes unused). Deno: the JavaScript runtime yt-dlp
+# uses to solve YouTube's "n" signature challenge for the web player clients (required alongside the PO
+# token, else only storyboards are offered). yt-dlp auto-detects deno on PATH.
 ARG DENO_VERSION=v2.9.1
 RUN apt-get update \
-    && apt-get install -y --no-install-recommends ffmpeg unzip curl ca-certificates \
+    && apt-get install -y --no-install-recommends ffmpeg intel-media-va-driver unzip curl ca-certificates \
     && curl -fsSL "https://github.com/denoland/deno/releases/download/${DENO_VERSION}/deno-x86_64-unknown-linux-gnu.zip" -o /tmp/deno.zip \
     && unzip -o /tmp/deno.zip -d /usr/local/bin \
     && chmod +x /usr/local/bin/deno \