fix(plex): a sidecar subtitle in a legacy code page rendered its accents as U+FFFD

A Hungarian .srt beside the video (Windows-1250) was decoded as UTF-8 with
errors=replace: one episode served 1391 U+FFFD and no accented letter.
decode_subtitle now lets a BOM decide, keeps valid UTF-8, and otherwise
reads the legacy code page of the stream's language (cp1250 for Central
European, cp1251 for Cyrillic, else cp1252) — on the ffmpeg path too.

The cache has no eviction, so its key is versioned (sub2_): a track cached
broken is read again once instead of being served for good.

backend/tests/test_plex_subtitle_prepare.py

@@ -311,8 +311,9 @@ def test_covered_ranges_coalesce_and_answer_membership():
 def test_a_cached_vtt_from_an_older_build_counts_as_complete(tmp_path):
     """Before this change the cache only ever held whole tracks, and none of them has a sidecar.
     Reading one as "covers nothing" would re-extract every already-cached track after the upgrade."""
-    (tmp_path / "sub_98614_0.vtt").write_text(_vtt(_cue("00:01.000", "00:02.000", "x")), "utf-8")
-    assert subtitles.read_meta("sub_98614_0") == ([[0.0, 1_000_000_000.0]], True)
+    key = subtitles.cache_key("98614", 0)
+    (tmp_path / f"{key}.vtt").write_text(_vtt(_cue("00:01.000", "00:02.000", "x")), "utf-8")
+    assert subtitles.read_meta(key) == ([[0.0, 1_000_000_000.0]], True)
 
 
 # --- the deep-review round: the seams between the window and the full pass ----------------------