Fix subtitle playback: vtt format, track elements, fast disk scan

- Convert subs to .vtt (was .srt which browsers don't support in <track>)
- Add GET /subtitle-files endpoint: instant disk scan for .vtt sidecar files,
  no yt-dlp call needed
- Inject <track> elements into the video player for each .vtt on disk;
  browser CC button appears automatically
- Before download: CC chip triggers YouTube availability check (slow, on demand)
- After download with subs: shows "CC ✓" — subtitles live in the player controls

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/api/index.js

@@ -91,6 +91,7 @@ export const createDownload = (youtube_video_id, quality, subtitle_langs) =>
   api.post("/downloads", { youtube_video_id, ...(quality ? { quality } : {}), ...(subtitle_langs ? { subtitle_langs } : {}) });
 export const getDownloads = () => api.get("/downloads");
 export const getAvailableSubs = (ytId) => api.get(`/videos/by-yt/${ytId}/subs`);
+export const getSubtitleFiles = (ytId) => api.get(`/videos/by-yt/${ytId}/subtitle-files`);
 export const getDownload = (id) => api.get(`/downloads/${id}`);
 export const deleteDownload = (id) => api.delete(`/downloads/${id}`);
 export const deleteAllDownloads = () => api.delete("/downloads/all");