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/routers/videos.py

@@ -662,10 +662,31 @@ def get_available_subs(
     youtube_video_id: str,
     current_user: User = Depends(get_current_user),
 ):
-    """Return subtitle languages available on YouTube for a video."""
+    """Return subtitle languages available on YouTube for a video (yt-dlp call, slow)."""
     return ytdlp.fetch_available_subs(youtube_video_id)
 
 
+@router.get("/by-yt/{youtube_video_id}/subtitle-files")
+def list_subtitle_files(
+    youtube_video_id: str,
+    current_user: User = Depends(get_current_user),
+):
+    """List .vtt subtitle files already on disk for a downloaded video (instant)."""
+    import re as _re
+    from pathlib import Path
+    from ..config import settings as _cfg
+    pat = _re.compile(rf'^{_re.escape(youtube_video_id)}\.(.+)\.vtt$')
+    subs = []
+    try:
+        for f in Path(_cfg.download_path).iterdir():
+            m = pat.match(f.name)
+            if m:
+                subs.append({"lang": m.group(1), "url": f"/files/{f.name}"})
+    except Exception:
+        pass
+    return sorted(subs, key=lambda s: s["lang"])
+
+
 @router.get("/by-yt/{youtube_video_id}/comments")
 def get_comments(
     youtube_video_id: str,

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 []
     )