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_encoding.py

@@ -0,0 +1,81 @@
+"""A sidecar subtitle in a legacy code page must render its accents, not U+FFFD.
+
+Found live on Arrow S03E15 (a Hungarian .srt beside the .mp4): the served WebVTT held 1391 U+FFFD and
+not one accented letter, because `_sidecar_vtt` decoded the bytes as UTF-8 with `errors="replace"`
+while the file was Windows-1250 (ő = 0xF5, ű = 0xFB). Legacy-code-page .srt files are the norm for
+Hungarian releases, and SubRip carries no charset of its own, so the stream's language is the only hint.
+No Postgres: Plex and ffmpeg are stubbed.
+"""
+
+from pathlib import Path
+from typing import cast
+
+import pytest
+from sqlalchemy.orm import Session
+
+from app.plex import subtitles
+
+HU = "1\r\n00:00:01,000 --> 00:00:02,000\r\nMiért nem szóltál? Ősz, űr.\r\n"
+FR = "1\r\n00:00:01,000 --> 00:00:02,000\r\nDéjà vu, garçon.\r\n"
+RU = "1\r\n00:00:01,000 --> 00:00:02,000\r\nПривет, мир.\r\n"
+
+
+class _Plex:
+    def __init__(self, raw: bytes):
+        self.raw = raw
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, *exc):
+        return False
+
+    def raw_get(self, key: str) -> bytes:
+        return self.raw
+
+
+def _sidecar(monkeypatch, raw: bytes, lang: str | None, codec: str = "srt") -> str:
+    monkeypatch.setattr(subtitles, "PlexClient", lambda db: _Plex(raw))
+    stream = {"key": "/library/streams/1", "codec": codec, "languageCode": lang}
+    return subtitles._sidecar_vtt(cast(Session, None), stream)
+
+
+@pytest.mark.parametrize(
+    ("text", "encoding", "lang"),
+    [
+        (HU, "cp1250", "hun"),  # the file that was found broken
+        (FR, "cp1252", "fre"),  # a Western legacy file
+        (RU, "cp1251", "rus"),  # a Cyrillic one
+        (HU, "utf-8", "hun"),  # already UTF-8: untouched
+        (HU, "utf-8-sig", "hun"),  # UTF-8 with a BOM
+        (HU, "utf-16", "hun"),  # UTF-16 (carries its BOM)
+    ],
+    ids=["cp1250-hu", "cp1252-fr", "cp1251-ru", "utf8", "utf8-bom", "utf16"],
+)
+def test_a_sidecar_srt_is_decoded_in_its_own_encoding(monkeypatch, text, encoding, lang):
+    vtt = _sidecar(monkeypatch, text.encode(encoding), lang)
+    assert "\ufffd" not in vtt
+    assert vtt.startswith("WEBVTT\n\n")
+    assert text.splitlines()[2] in vtt
+
+
+def test_a_non_srt_sidecar_reaches_ffmpeg_as_utf8(monkeypatch):
+    seen: dict[str, bytes] = {}
+
+    def fake_ffmpeg(src: str, sub_ord):
+        seen["bytes"] = Path(src).read_bytes()
+        return "WEBVTT\n"
+
+    monkeypatch.setattr(subtitles, "ffmpeg_to_vtt", fake_ffmpeg)
+    _sidecar(monkeypatch, "[Events]\nDialogue: Ősz\n".encode("cp1250"), "hun", codec="ass")
+    assert seen["bytes"].decode("utf-8") == "[Events]\nDialogue: Ősz\n"
+
+
+def test_a_track_cached_before_the_encoding_fix_is_not_served(tmp_path, monkeypatch):
+    """The cache has no eviction, so a mis-decoded track would have stayed broken for good: the key
+    is versioned, and a track cached under the old one is read again (once) instead of served."""
+    monkeypatch.setattr(subtitles, "CACHE_DIR", tmp_path)
+    (tmp_path / "sub_14881_1.vtt").write_text(
+        "WEBVTT\n\n1\n00:00:01.000 --> 00:00:02.000\nAz�rt\n", "utf-8"
+    )
+    assert subtitles.cached_track("14881", 1)[0] is None