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.

backend/tests/test_plex_playback_class.py

@@ -451,3 +451,45 @@ def test_facet_playable_self_excludes_the_active_selection(db, monkeypatch):
     assert resp.status_code == 200, resp.text
     counts = {p["value"]: p["count"] for p in resp.json()["playable"]}
     assert counts == {"direct": 1, "remux": 1, "transcode_hw": 1, "transcode_sw": 1}
+
+
+def test_active_playable_narrows_the_other_facets(db, monkeypatch):
+    # The flip side of self-exclusion: an active playable class DOES narrow the OTHER facet groups (so
+    # e.g. the year range reflects only the selected class). Seed a Direct movie (2000) and a
+    # software-transcode movie (2020); selecting playable=direct must collapse the year facet to 2000.
+    lib = PlexLibrary(plex_key="1", title="Movies", kind="movie", enabled=True)
+    db.add(lib)
+    db.flush()
+    db.add_all(
+        [
+            PlexItem(
+                rating_key="d2000",
+                library_id=lib.id,
+                kind="movie",
+                title="Direct2000",
+                playable="direct",
+                year=2000,
+            ),
+            PlexItem(
+                rating_key="s2020",
+                library_id=lib.id,
+                kind="movie",
+                title="SW2020",
+                playable="transcode",
+                codec_video="mpeg4",
+                transcode_hw=False,
+                year=2020,
+            ),
+        ]
+    )
+    user = User(email="pnarrow@example.com")
+    db.add(user)
+    db.commit()
+    client = _client(db, user, monkeypatch)
+    try:
+        wide = client.get("/api/plex/facets?scope=movie").json()
+        narrow = client.get("/api/plex/facets?scope=movie&playable=direct").json()
+    finally:
+        app.dependency_overrides.clear()
+    assert (wide["year_min"], wide["year_max"]) == (2000, 2020)  # both classes span the range
+    assert (narrow["year_min"], narrow["year_max"]) == (2000, 2000)  # narrowed to the Direct subset