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/routers/videos.py

@@ -666,6 +666,22 @@ def get_available_subs(
     return ytdlp.fetch_available_subs(youtube_video_id)
 
 
+@router.post("/by-yt/{youtube_video_id}/download-subs")
+def download_subs(
+    youtube_video_id: str,
+    body: dict,
+    current_user: User = Depends(get_current_user),
+):
+    """Download subtitle file(s) only for an already-downloaded video."""
+    langs = (body.get("subtitle_langs") or "").strip()
+    if not langs:
+        raise HTTPException(status_code=400, detail="subtitle_langs required")
+    ok = ytdlp.download_subs_only(youtube_video_id, langs)
+    if not ok:
+        raise HTTPException(status_code=500, detail="Subtitle download failed")
+    return {"ok": True}
+
+
 @router.get("/by-yt/{youtube_video_id}/subtitle-files")
 def list_subtitle_files(
     youtube_video_id: str,