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/routers/downloads.py

@@ -19,6 +19,7 @@ router = APIRouter()
 class DownloadRequest(BaseModel):
     youtube_video_id: str
     quality: Optional[str] = None
+    subtitle_langs: Optional[str] = None  # overrides user setting when provided
 
 
 TRASH_TTL_DAYS = 7
@@ -127,7 +128,10 @@ def create_download(
     user_settings = db.query(UserSettings).filter_by(user_id=current_user.id).first()
     default_quality = user_settings.preferred_quality if user_settings else "best"
     quality = body.quality if body.quality in ytdlp.QUALITY_FORMATS else default_quality
-    subtitle_langs = (user_settings.subtitle_langs or "") if user_settings else ""
+    if body.subtitle_langs is not None:
+        subtitle_langs = body.subtitle_langs.strip()
+    else:
+        subtitle_langs = (user_settings.subtitle_langs or "") if user_settings else ""
 
     _DL_SELECT = """
         SELECT d.id, d.status, d.progress_percent, d.resolution,