Add scheduled sync, disk space awareness, and subtitle downloads

- auto-sync daemon: background thread checks every hour and syncs followed
  channels for users with sync_interval_hours set (6/12/24h options)
- disk stats: /api/stats now returns total/used/free/download bytes;
  Stats page shows a disk usage bar
- subtitles: subtitle_langs setting (e.g. "en,sv") passed through all
  download paths; yt-dlp writes .srt files alongside the video
- Settings page: sync interval dropdown + subtitle languages input

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

backend/routers/channels.py

@@ -147,6 +147,7 @@ def _index_channel_task(channel_id: int, user_id: int):
 
             if channel_auto:
                 quality = user_settings.preferred_quality if user_settings else "best"
+                subtitle_langs = (user_settings.subtitle_langs or "") if user_settings else ""
                 from ..routers.downloads import _on_progress, _on_complete, _on_error
                 for yt_id, vid_id in new_video_ids:
                     existing_dl = db.query(Download).filter_by(
@@ -159,7 +160,7 @@ def _index_channel_task(channel_id: int, user_id: int):
                         import threading
                         t = threading.Thread(
                             target=ytdlp.start_download,
-                            args=(yt_id, dl.id, _on_progress, _on_complete, _on_error, quality),
+                            args=(yt_id, dl.id, _on_progress, _on_complete, _on_error, quality, subtitle_langs),
                             daemon=True,
                         )
                         t.start()