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/settings.py

@@ -34,6 +34,8 @@ class SettingsOut(BaseModel):
     feed_weight_affinity: float = 5.0
     feed_weight_channel: float = 5.0
     use_oauth2: bool = False
+    sync_interval_hours: int = 0
+    subtitle_langs: str = ""
 
     model_config = {"from_attributes": True}
 
@@ -55,6 +57,8 @@ class SettingsPatch(BaseModel):
     feed_weight_affinity: Optional[float] = Field(None, ge=0.0, le=10.0)
     feed_weight_channel: Optional[float] = Field(None, ge=0.0, le=10.0)
     use_oauth2: Optional[bool] = None
+    sync_interval_hours: Optional[int] = Field(None, ge=0, le=168)
+    subtitle_langs: Optional[str] = None
 
 
 def _get_or_create(db: Session, user_id: int) -> UserSettings:
@@ -123,6 +127,10 @@ def update_settings(
     if body.use_oauth2 is not None:
         s.use_oauth2 = body.use_oauth2
         ytdlp.set_oauth2(body.use_oauth2)
+    if body.sync_interval_hours is not None:
+        s.sync_interval_hours = body.sync_interval_hours
+    if body.subtitle_langs is not None:
+        s.subtitle_langs = body.subtitle_langs.strip()
 
     db.commit()
     db.refresh(s)