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/main.py

@@ -71,6 +71,8 @@ def on_startup():
         "ALTER TABLE user_settings ADD COLUMN feed_weight_affinity REAL DEFAULT 5.0",
         "ALTER TABLE user_settings ADD COLUMN feed_weight_channel REAL DEFAULT 5.0",
         "ALTER TABLE user_settings ADD COLUMN use_oauth2 INTEGER DEFAULT 0",
+        "ALTER TABLE user_settings ADD COLUMN sync_interval_hours INTEGER DEFAULT 0",
+        "ALTER TABLE user_settings ADD COLUMN subtitle_langs TEXT DEFAULT ''",
         """CREATE TABLE IF NOT EXISTS search_history (
             id INTEGER PRIMARY KEY AUTOINCREMENT,
             user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
@@ -151,9 +153,47 @@ def on_startup():
 
     # Backfill descriptions for videos that don't have them yet (runs in background)
     import threading
-    from .routers.channels import _enrich_missing_task
+    from .routers.channels import _enrich_missing_task, _index_channels_batch
     threading.Thread(target=_enrich_missing_task, args=(3,), daemon=True).start()
 
+    def _auto_sync_daemon():
+        import time
+        from datetime import datetime, timedelta
+        from sqlalchemy import text as _text
+        while True:
+            time.sleep(3600)
+            try:
+                db = SessionLocal()
+                try:
+                    users_due = db.execute(
+                        _text("SELECT user_id, sync_interval_hours FROM user_settings WHERE sync_interval_hours > 0")
+                    ).mappings().all()
+                    for row in users_due:
+                        uid = row["user_id"]
+                        cutoff = datetime.utcnow() - timedelta(hours=row["sync_interval_hours"])
+                        ch_ids = [
+                            r["id"] for r in db.execute(
+                                _text("""
+                                    SELECT c.id FROM channels c
+                                    JOIN user_channels uc ON c.id = uc.channel_id
+                                    WHERE uc.user_id = :uid AND uc.status = 'followed'
+                                      AND (c.crawled_at IS NULL OR c.crawled_at < :cutoff)
+                                    ORDER BY COALESCE(c.crawled_at, '1970-01-01') ASC
+                                """),
+                                {"uid": uid, "cutoff": cutoff},
+                            ).mappings().all()
+                        ]
+                        if ch_ids:
+                            threading.Thread(
+                                target=_index_channels_batch, args=(ch_ids, uid), daemon=True
+                            ).start()
+                finally:
+                    db.close()
+            except Exception:
+                pass
+
+    threading.Thread(target=_auto_sync_daemon, daemon=True).start()
+
 
 @app.get("/api/health")
 def health():