feat(plex): opt into hardware.gpu: intel-quicksync, and write the runbook for the step GitOps cannot reach

Plex has been transcoding in software on hardware that can do it in the iGPU. This
opts its ServiceDefinition into the `hardware` capability from #27 and re-renders.

catalog/plex.yaml gains four lines of YAML and the comment that explains them:

    hardware:
      gpu: intel-quicksync

which renders exactly two things into apps/wave-0/plex.yaml and nothing else — a
`dri` hostPath volume (/dev/dri, type: DirectoryOrCreate) and a read-write /dev/dri
volumeMount on the app container. Whole-tree render diff: 6 added lines, one file.

The comment records what is not obvious from the four lines: the device exists on
ALL THREE prod nodes with i915 bound (so no nodeSelector is rendered and none is
wanted); why `supplementalGroups` would be INERT rather than a hardening (the
image's cont-init hook adds the plex user to the device's group inside the
container, and s6-setuidgid then REPLACES the process group list from /etc/group,
discarding anything Kubernetes supplied); and why `DirectoryOrCreate` is
load-bearing (apps-wave-0 is wait:true with apps-wave-1 depending on it, so a
failed mount would stall the whole app tier for a feature that is not).

THE FEATURE IS INERT UNTIL A HUMAN CLICKS A BOX, and that is the part this commit
is really about. Mounting the device does not enable hardware transcoding; Plex's
`HardwareAcceleratedCodecs` preference does, and it is ABSENT from Preferences.xml
— the toggle has never been on. It cannot be automated: a bounded grep across the
running image's entire init surface (/etc/cont-init.d, /etc/services.d,
/etc/fix-attrs.d, /plex-common.sh, /usr/local/bin) found ZERO PLEX_PREFERENCE*
support, and the six setPref keys the image does write are all unrelated. So
runbooks/plex-quicksync.md is the procedure, registered in the configMapGenerator
(a runbook in git with no generator entry is reviewed, merged, and still a 404) and
indexed in runbooks/README.md. It covers the GitOps/manual boundary, the toggle and
the real preference keys (including the widely-cited one that does not exist and
the upstream misspelling nobody should "fix"), verification via `Transcode (hw)` on
the Plex dashboard — stated plainly as the ONLY proof, because Plex renders no
ServiceMonitor and an alert could never fire — the device-cgroup failure to expect
first with its Intel-device-plugin fallback, and the floating-`:latest` caveat.

Stacked on #27 (feat/sd-hardware-capability), which is open and unmerged; that
branch's commit is in this range by design and this must merge after it.

NOT VERIFIED, and the handoff says so at length: hardware transcoding has not been
observed working. Whether containerd's device cgroup lets a non-privileged pod
open() a bind-mounted char device is an open question that only deploying settles,
and the Plex-side toggle was still unset at the time of writing. A green gate
proves the manifest renders; it proves nothing about transcoding.

Gate (pinned toolbox, worktree): rc 3 — UNDETERMINED, not green. Passed:
decisions-log, lint, l0, render-golden, l1, approval-policy. Skipped (the known
local ceiling): spec-immutability (no sibling spec repo), gitea-settings (no
SETTINGS_API_TOKEN), conformance (CLUSTER_AVAILABLE unset, so L2/L3 did not run).
No fourth skip, nothing failed.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

runbooks/README.md

@@ -115,6 +115,14 @@ Each affected runbook says so on its own page and gives the by-hand equivalent.
   plex.tv/claim — so it is a timed sequence, scripted as `infra/plex/claim-ship.sh`:
   token → `sops` → PR → gate → merge → **forced** `flux reconcile` → pod roll (~2m45s measured).
   Includes why a changed Secret does not restart anything, and why an expired token is harmless.
+- **`plex-quicksync`** — hardware transcoding on the host iGPU. GitOps puts `/dev/dri` in the pod
+  (`hardware.gpu: intel-quicksync`); **a human still has to tick Settings → Transcoder → "Use hardware
+  acceleration when available"**, and no environment variable can do it (the image supports no
+  `PLEX_PREFERENCE*`, verified). The page names the real preference key (`HardwareAcceleratedCodecs`,
+  read out of the binary), says which widely-cited key **does not exist**, and is explicit that the
+  **only** proof is `Transcode (hw)` on the Plex dashboard — Plex renders no ServiceMonitor, so no
+  alert could ever fire. Also: the device-cgroup failure to expect first and its device-plugin
+  fallback, and why a `:latest` pull can silently break it.
 - **`oauth2-proxy-session`** — a gated UI loads but its saves quietly do nothing, and you are never
   re-prompted to log in. Tells the three stacked auth layers apart from the sidecar's own access log
   (the four lines that matter, including the upstream `-` that means the proxy answered a request the