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/stats.py

@@ -1,8 +1,12 @@
+import os
+import shutil
+
 from fastapi import APIRouter, Depends, HTTPException
 from sqlalchemy.orm import Session
 from sqlalchemy import text
 
 from ..auth_utils import get_current_user
+from ..config import settings
 from ..database import get_db
 from ..models import User, UserTagAffinity
 
@@ -120,6 +124,15 @@ def get_stats(
         {"uid": uid},
     ).mappings().first()
 
+    try:
+        disk = shutil.disk_usage(settings.download_path)
+        download_bytes = sum(
+            e.stat().st_size for e in os.scandir(settings.download_path) if e.is_file()
+        )
+    except Exception:
+        disk = None
+        download_bytes = 0
+
     return {
         "total_watched": totals["total_watched"] or 0,
         "total_watch_seconds": totals["total_watch_seconds"] or 0,
@@ -141,6 +154,12 @@ def get_stats(
         "total_liked": liked_count["n"] or 0,
         "top_categories": [dict(r) for r in top_categories],
         "taste_profile": [dict(r) for r in taste_profile],
+        "disk": {
+            "total_bytes": disk.total if disk else None,
+            "free_bytes": disk.free if disk else None,
+            "used_bytes": disk.used if disk else None,
+            "download_bytes": download_bytes,
+        },
     }