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.

frontend/src/lib/releaseNotes.ts

@@ -14,6 +14,14 @@ export interface ReleaseEntry {
 }
 
 export const RELEASE_NOTES: ReleaseEntry[] = [
+  {
+    version: "0.28.3",
+    date: "2026-07-06",
+    summary: "Fix: Plex subtitles that wouldn't play — now instant, no restart.",
+    features: [
+      "Plex subtitles: fixed playback failing (\"Playback couldn't start\") when picking a subtitle on films whose subtitles are external sidecar files (very common) — those aren't inside the video, so the old approach couldn't mux them. Subtitles are now served as standalone tracks the browser overlays, so turning one on (or switching) is instant and no longer restarts the video. Image-based subtitles (Blu-ray/DVD) are hidden for now (they can't be shown as text yet).",
+    ],
+  },
   {
     version: "0.28.2",
     date: "2026-07-06",