Add per-video subtitle language picker on Watch page

- fetch_available_subs() queries yt-dlp for manual + auto-generated
  subtitle langs available on YouTube for any given video
- GET /api/videos/by-yt/{ytId}/subs exposes this to the frontend
- DownloadRequest now accepts subtitle_langs to override the global
  setting on a per-download basis
- Watch page fetches available subtitle langs on load (in parallel),
  shows a CC dropdown with manual langs + auto-generated langs labeled
  "(auto)"; selected lang is passed through to the download

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

backend/services/ytdlp.py

@@ -384,6 +384,36 @@ def fetch_channel_links(channel_id: str) -> list[str]:
     return list(channel_ids)
 
 
+def fetch_available_subs(video_id: str) -> dict:
+    """Return subtitle languages available on YouTube for a video.
+
+    Returns {"manual": [...], "auto": [...]} where both are sorted lists of
+    BCP-47 lang codes. Manual = human-made; auto = auto-generated captions.
+    """
+    url = f"https://www.youtube.com/watch?v={video_id}"
+    base_cmd = ["yt-dlp", url, "--dump-json", "--no-download", "--no-playlist"]
+    cookie_args = _cookie_args()
+    stdout, _, code = _run([*base_cmd, *cookie_args], timeout=30)
+    if code != 0 and cookie_args:
+        stdout, _, code = _run(base_cmd, timeout=30)
+
+    for line in stdout.splitlines():
+        line = line.strip()
+        if not line:
+            continue
+        try:
+            info = json.loads(line)
+            manual = sorted(info.get("subtitles") or {})
+            auto = sorted(set(
+                lang for lang in (info.get("automatic_captions") or {})
+                if not lang.endswith("-orig")
+            ))
+            return {"manual": manual, "auto": auto}
+        except json.JSONDecodeError:
+            continue
+    return {"manual": [], "auto": []}
+
+
 def fetch_video_comments(youtube_video_id: str, max_comments: int = 20) -> list[dict]:
     """Fetch top comments via yt-dlp CLI writing to a temp file. Returns empty list on failure."""
     import os