fix(plex): re-probe on file replace, HU parity, public class const, playback-class test gaps

Deep-review round 1 on feature/plex-playback-class-filter:

- Sticky watermark (medium): _apply_item reset the bit-depth probe watermark
  only on a codec-string change, so a same-codec re-encode (8-bit->10-bit HEVC)
  kept a stale video_bit_depth/transcode_hw forever (the backfill only re-probes
  media_probed_at IS NULL rows). Also reset when the Plex Part key changes (the
  file was replaced), not just the codec.

- i18n parity (medium): the HU playback-class labels were mixed-language (only
  'direct' translated). Translated remux/transcode_hw/transcode_sw and the
  tooltips consistently to Hungarian (Kozvetlen streameles / Atkodolas).

- Layering (low): _PLAYBACK_CLASSES was module-private yet imported across into
  routes.plex.browse; renamed to public PLAYBACK_CLASSES (it is the shared
  source of truth for facet order + zero-fill).

- a11y (low): the browse-card playback-class dot conveyed meaning only via a
  mouse-hover title; added role=img + aria-label so SR/touch users get the class.

- tests: the NULL/unprobed transcode_hw -> SW fold (filter + facet), facet
  self-exclusion under an active playable filter, the migration codec-set ==
  HW_DECODABLE_CODECS drift guard, the probe-batch bound + oldest-first order,
  the transient probe-failure retry path, and the same-codec re-encode re-probe.

Backend gate green (ruff + full pytest); FE gate green (tsc/lint/knip/prettier/
vitest). NOTE: the facet-vs-runtime disagreement for 10-bit HEVC (C1) is the
known merge-order constraint — this branch's runtime is codec-only; the facet
becomes truthful once BUG1/tonemap's bit-depth-aware routing lands with it. Left
as a sequencing constraint, not code-changed here.

frontend/src/components/plexCards.tsx

@@ -100,6 +100,8 @@ export function PlexPosterCard({
             const cls = playableClass(card.playable, card.transcode_hw);
             return (
               <span
+                role="img"
+                aria-label={t(`plex.playable.${cls}`)}
                 className={`absolute top-1.5 left-1.5 z-20 w-2 h-2 rounded-full ring-2 ring-black/40 ${PLAYABLE_TINT[cls] ?? "bg-gray-400"}`}
                 title={t(`plex.playable.${cls}`)}
               />