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/client.py

@@ -113,6 +113,16 @@ class PlexClient:
         items = mc.get("Metadata", []) or []
         return items[0] if items else None
 
+    def raw_get(self, path: str) -> bytes:
+        """Raw bytes of an arbitrary Plex resource path (e.g. an external subtitle stream `key` like
+        ``/library/streams/553184``). Keeps the admin token server-side."""
+        try:
+            r = self._http.get(f"{self.base}{path}")
+            r.raise_for_status()
+        except httpx.HTTPError as e:
+            raise PlexError(str(e)) from e
+        return r.content
+
     def image_bytes(
         self, image_path: str, width: int | None = None, height: int | None = None
     ) -> tuple[bytes, str]: