fix(plex): deep-review follow-ups on E6 transcode + player polish

Eight verified findings from the sprint deep-review:

- Transcode cap: _enforce_cap now counts only transcode sessions, so a
  live remux (stream-copy) stream can no longer be LRU-evicted mid-play —
  matching the config comment + the HU/EN Config hint that promise remux
  is uncapped.
- Player Back: the browser/mouse Back interceptor (BackClose) now also
  mounts for the stats overlay and up-next card, so Back closes them first
  instead of ejecting the whole player (keyboard/Esc already did).
- Honest cold-start: loadSession clears sessionIsTranscode up front, so a
  transcode->direct/remux switch no longer flashes a stale "Preparing
  stream..." during the session request.
- Stats overlay a11y: the close (X) buttons in the stats, help and info
  overlays now read as "Close"/"Bezaras" (new plex.player.close key), not
  "Back"; dropped aria-live from the per-second stat grid so a screen
  reader is not flooded with telemetry.

Tests: route-level transcode routing (transcode item -> HLS re-encode
session, remux stays copy, direct opens none); start_session threading the
preset/crf/max_height knobs + hw/preexec into the ffmpeg argv; UpNextCard
countdown auto-advance, Play-now, Cancel and the S?-E? heading fallback.

Also brings UpNextCard.tsx / plexStats.ts / plexStats.test.ts to prettier.

frontend/src/components/PlexStatsOverlay.tsx

@@ -109,13 +109,16 @@ export function PlexStatsOverlay({
         </span>
         <button
           onClick={onClose}
-          aria-label={t("plex.player.back")}
+          aria-label={t("plex.player.close")}
           className="rounded p-0.5 text-white/60 hover:bg-white/10 hover:text-white"
         >
           <X className="h-4 w-4" />
         </button>
       </div>
-      <dl aria-live="polite" className="grid grid-cols-[auto_1fr] gap-x-3 gap-y-1 text-2xs">
+      {/* No aria-live here: the rows re-sample every second, so a live region would make a screen
+          reader announce the changing values continuously and drown out real player announcements.
+          The overlay is a role="region" (labelled above) a user reads on demand, not a status feed. */}
+      <dl className="grid grid-cols-[auto_1fr] gap-x-3 gap-y-1 text-2xs">
         {rows.map((r) => (
           <div key={r.key} className="contents">
             <dt className="text-white/55">{t(r.labelKey)}</dt>