fix: use ffmpeg downloader for proper progress reporting

VOE provider returns HLS streams (.m3u8) which previously used yt-dlp's
native HLS downloader. That downloader does not report downloaded_bytes/
total_bytes in progress hooks, causing only 0% and 100% to ever show on
the queue page. Explicitly set 'downloader: ffmpeg' so progress updates
with actual byte counts are broadcast, enabling intermediate percentages
(25%, 50%, 75%, etc.) to display correctly.

src/server/providers/aniworld_provider.py

@@ -543,6 +543,7 @@ class AniworldLoader(Loader):
                 self.events.download_progress(d)
 
             ydl_opts = {
+                'downloader': 'ffmpeg',  # Use ffmpeg for proper progress reporting
                 'fragment_retries': float('inf'),
                 'outtmpl': temp_path,
                 'quiet': True,