Add subtitle-only download for already-downloaded videos

- download_subs_only(): yt-dlp --skip-download to fetch just .vtt sidecar
- POST /by-yt/{ytId}/download-subs endpoint
- CC chip now visible on downloaded videos; clicking checks YouTube,
  shows lang picker with "Add subtitles" button separate from re-download

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

backend/services/ytdlp.py

@@ -384,6 +384,22 @@ def fetch_channel_links(channel_id: str) -> list[str]:
     return list(channel_ids)
 
 
+def download_subs_only(video_id: str, subtitle_langs: str) -> bool:
+    """Download subtitle files only (no video) for an already-downloaded video."""
+    url = f"https://www.youtube.com/watch?v={video_id}"
+    output_template = str(Path(settings.download_path) / f"{video_id}.%(ext)s")
+    _, _, code = _run([
+        "yt-dlp", url,
+        "--skip-download", "--no-playlist",
+        "--write-subs", "--write-auto-subs",
+        "--sub-langs", subtitle_langs,
+        "--convert-subs", "vtt",
+        "-o", output_template,
+        *_cookie_args(),
+    ], timeout=60)
+    return code == 0
+
+
 def fetch_available_subs(video_id: str) -> dict:
     """Return subtitle languages available on YouTube for a video.