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/services/ytdlp.py

@@ -621,17 +621,20 @@ def start_download(
     on_complete: Any,
     on_error: Any,
     quality: str = "best",
+    subtitle_langs: str = "",
 ) -> None:
     """Start yt-dlp download in a background thread.
 
-    Uses a single progressive MP4 format so the file is playable as it downloads.
     --no-part writes directly to the final filename (no .part rename at the end).
     """
     url = f"https://www.youtube.com/watch?v={video_id}"
-    # Predictable output path — lets the player start before download finishes
     output_template = str(Path(settings.download_path) / f"{video_id}.%(ext)s")
 
     fmt = QUALITY_FORMATS.get(quality, QUALITY_FORMATS["best"])
+    subtitle_args = (
+        ["--write-subs", "--write-auto-subs", "--sub-langs", subtitle_langs, "--convert-subs", "srt"]
+        if subtitle_langs else []
+    )
 
     def _run_download():
         with _SEMAPHORE:
@@ -645,6 +648,7 @@ def start_download(
                     "--no-part", "--no-mtime",
                     "-o", output_template,
                     "--newline", "--progress", "--no-colors",
+                    *subtitle_args,
                     *cookie_args,
                 ],
                 stdout=subprocess.PIPE,