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>

backend/services/ytdlp.py

@@ -662,7 +662,7 @@ def start_download(
 
     fmt = QUALITY_FORMATS.get(quality, QUALITY_FORMATS["best"])
     subtitle_args = (
-        ["--write-subs", "--write-auto-subs", "--sub-langs", subtitle_langs, "--convert-subs", "srt"]
+        ["--write-subs", "--write-auto-subs", "--sub-langs", subtitle_langs, "--convert-subs", "vtt"]
         if subtitle_langs else []
     )