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/models.py

@@ -122,6 +122,8 @@ class UserSettings(Base):
     feed_weight_affinity = Column(Float, default=5.0)  # 0–10
     feed_weight_channel = Column(Float, default=5.0)   # 0–10
     use_oauth2 = Column(Boolean, default=False)
+    sync_interval_hours = Column(Integer, default=0)   # 0 = disabled, 6/12/24 = auto-sync interval
+    subtitle_langs = Column(String, default="")        # "" = disabled, "en", "en,sv", etc.
 
 
 class DiscoveryQueue(Base):