fix(plex): play subtitles as WebVTT tracks (external sidecar subs no longer crash)

Selecting a subtitle restarted the HLS session with `-map 0:s:{ord}`, assuming an
EMBEDDED stream. Films whose subs are external sidecar .srt files (Plex reports
them, but they aren't in the mkv) matched no stream; the master-playlist's
declared subtitle group then made ffmpeg fail → "Playback couldn't start".

Subtitles now go through a new GET /api/plex/subtitle/{rk}/{ord} → text/vtt
(external subs fetched from Plex via the stream key + SRT→VTT; embedded text subs
extracted with ffmpeg; image subs → 415), served as native <video><track> that
the browser overlays. So choosing/switching a subtitle is instant with NO session
restart, and stream.py drops all subtitle muxing (`-sn`, no master playlist).
Image-based subs (PGS/VobSub) are marked text=false and hidden in the picker.
Verified on prod's Nymphomaniac Vol. II: HU sidecar → 1693 WebVTT cues, no crash.

backend/app/plex/stream.py

@@ -48,24 +48,17 @@ class HlsSession:
         self.last_access = time.time()
 
 
-def _ffmpeg_cmd(
-    src: Path, start_s: float, out_dir: Path, audio_ord: int | None, sub_ord: int | None
-) -> list[str]:
+def _ffmpeg_cmd(src: Path, start_s: float, out_dir: Path, audio_ord: int | None) -> list[str]:
     args = ["ffmpeg", "-nostdin", "-loglevel", "error"]
     if start_s > 0:
         args += ["-ss", f"{start_s:.3f}"]  # fast keyframe seek before -i
     args += ["-i", str(src)]
-    # Video always stream-copied. The selected audio track (default = first) is transcoded to AAC.
-    # A selected subtitle track is muxed in as a WebVTT rendition (ffmpeg aligns it to this session's
-    # timeline, so it stays in sync even after a seek-restart); no selection → drop subtitles.
-    args += ["-map", "0:v:0", "-map", f"0:a:{audio_ord if audio_ord is not None else 0}?"]
-    if sub_ord is not None:
-        args += ["-map", f"0:s:{sub_ord}?", "-c:s", "webvtt"]
-    else:
-        args += ["-sn"]
+    # Video always stream-copied; the selected audio track (default = first) is transcoded to AAC.
+    # Subtitles are NOT muxed here — they're served as standalone WebVTT tracks (GET /subtitle) that
+    # the browser overlays, so choosing a subtitle no longer restarts this session (and external
+    # sidecar subs, which aren't in the file, work — the old `-map 0:s` broke on those).
+    args += ["-map", "0:v:0", "-map", f"0:a:{audio_ord if audio_ord is not None else 0}?", "-sn"]
     args += ["-c:v", "copy", "-c:a", "aac", "-ac", "2", "-b:a", "192k"]
-    if sub_ord is not None:
-        args += ["-c:s", "webvtt"]
     args += [
         "-f", "hls",
         "-hls_time", str(_SEG_SECONDS),
@@ -77,10 +70,6 @@ def _ffmpeg_cmd(
         "-hls_segment_type", "mpegts",
         "-hls_flags", "independent_segments+temp_file",
     ]
-    if sub_ord is not None:
-        # A subtitle rendition needs a MASTER playlist tying the video variant to the WebVTT group
-        # (ffmpeg generates the subtitle segments per-session, so they stay in sync after a seek).
-        args += ["-master_pl_name", "master.m3u8", "-var_stream_map", "v:0,a:0,s:0,sgroup:subs"]
     args += ["-hls_segment_filename", str(out_dir / "seg_%d.ts"), str(out_dir / "index.m3u8")]
     return args
 
@@ -111,10 +100,10 @@ def start_session(
     item: PlexItem,
     start_s: float,
     audio_ord: int | None = None,
-    sub_ord: int | None = None,
 ) -> HlsSession | None:
-    """(Re)start the HLS remux for an item at the given offset, with an optional selected audio /
-    subtitle track (ordinals among their stream type). Returns None if the local file can't be read."""
+    """(Re)start the HLS remux for an item at the given offset, with an optional selected audio track
+    (ordinal among audio streams). Subtitles are separate WebVTT tracks (not muxed here). Returns
+    None if the local file can't be read."""
     src = paths.local_media_path(db, item.file_path)
     if src is None:
         return None
@@ -124,21 +113,18 @@ def start_session(
         old = _sessions.pop(key, None)
         if old is not None:
             _kill(old)
-        directory = _HLS_ROOT / f"{key}_{int(start_s)}_{audio_ord}_{sub_ord}"
+        directory = _HLS_ROOT / f"{key}_{int(start_s)}_{audio_ord}"
         shutil.rmtree(directory, ignore_errors=True)
         directory.mkdir(parents=True, exist_ok=True)
         proc = subprocess.Popen(
-            _ffmpeg_cmd(src, start_s, directory, audio_ord, sub_ord),
+            _ffmpeg_cmd(src, start_s, directory, audio_ord),
             stdout=subprocess.DEVNULL,
             stderr=subprocess.DEVNULL,
         )
-        entry = "master.m3u8" if sub_ord is not None else "index.m3u8"
-        s = HlsSession(key, directory, proc, start_s, entry)
+        s = HlsSession(key, directory, proc, start_s, "index.m3u8")
         _sessions[key] = s
         _enforce_cap()
-        log.info(
-            "plex hls session start key=%s start=%.1f audio=%s sub=%s", key, start_s, audio_ord, sub_ord
-        )
+        log.info("plex hls session start key=%s start=%.1f audio=%s", key, start_s, audio_ord)
         return s