fix(plex): gate the part-key re-probe to single-version items; drop dead i18n key; cover cross-facet narrowing

Deep-review round 2 on the round-1 fixes (all low):

- part_key churn (state): round 1's part-key reset re-probed EVERY sync for a
  multi-version item (an original + a Plex 'Optimized' copy), because
  _media_facts reads media[0]/part[0] whose order Plex doesn't guarantee stable,
  oscillating transcode_hw between the two versions. Gate the part-key signal on
  a single-version item (len(Media)<=1); codec-change reset still applies to all.
  A multi-version same-codec re-encode not self-healing is an acceptable rare
  edge; a per-sync re-probe loop is not.

- dead i18n key: this diff's playableClass() collapses to transcode_hw/sw, so the
  bare plex.playable.transcode key it orphaned is now unused (knip can't see
  dynamic keys). Dropped from both locales, parity kept.

- test: cross-facet narrowing by an active playable class (a co-facet's year
  range collapses to the selected subset) — the flip side of self-exclusion,
  previously unpinned.

The aria-label+title double-announcement on the card dot is a deliberate accepted
trade-off (reliable SR name + mouse tooltip). Backend + FE gates green.

frontend/src/i18n/locales/en/plex.json

@@ -293,7 +293,6 @@
   "playable": {
     "direct": "Plays directly in the browser",
     "remux": "Direct Stream — a light remux (no video re-encode)",
-    "transcode": "Needs transcoding (heavier)",
     "transcode_hw": "Transcoded on the hardware encoder",
     "transcode_sw": "Transcoded in software (heaviest)"
   },

frontend/src/i18n/locales/hu/plex.json

@@ -293,7 +293,6 @@
   "playable": {
     "direct": "Közvetlenül játszható a böngészőben",
     "remux": "Közvetlen streamelés — könnyű remux (nincs videó-újrakódolás)",
-    "transcode": "Átkódolás szükséges (nehezebb)",
     "transcode_hw": "Hardveres kódolóval átkódolva",
     "transcode_sw": "Szoftveresen átkódolva (legnehezebb)"
   },