fix: use YouTube-provided thumbnail URLs instead of hardcoded hq720.jpg

Videos without hq720.jpg thumbnails caused mass 404 errors.
Now preserves the actual thumbnail URL from YouTube's API response,
falls back to hqdefault.jpg only when no thumbnail is provided.
Also picks highest quality thumbnail from API (thumbnails[-1])
and adds progressive fallback for subscription/download functions.

youtube/playlist.py

@@ -106,8 +106,8 @@ def get_playlist_page():
     for item in info.get('items', ()):
         util.prefix_urls(item)
         util.add_extra_html_info(item)
-        if 'id' in item:
-            item['thumbnail'] = f"{settings.img_prefix}https://i.ytimg.com/vi/{item['id']}/hq720.jpg"
+        if 'id' in item and not item.get('thumbnail'):
+            item['thumbnail'] = f"{settings.img_prefix}https://i.ytimg.com/vi/{item['id']}/hqdefault.jpg"
 
         item['url'] += '&list=' + playlist_id
         if item['index']: