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/yt_data_extract/common.py

@@ -369,9 +369,9 @@ def extract_item_info(item, additional_info={}):
         ['detailedMetadataSnippets', 0, 'snippetText'],
     ))
     info['thumbnail'] = normalize_url(multi_deep_get(item,
-        ['thumbnail', 'thumbnails', 0, 'url'],      # videos
-        ['thumbnails', 0, 'thumbnails', 0, 'url'],  # playlists
-        ['thumbnailRenderer', 'showCustomThumbnailRenderer', 'thumbnail', 'thumbnails', 0, 'url'], # shows
+        ['thumbnail', 'thumbnails', -1, 'url'],      # videos (highest quality)
+        ['thumbnails', 0, 'thumbnails', -1, 'url'],  # playlists
+        ['thumbnailRenderer', 'showCustomThumbnailRenderer', 'thumbnail', 'thumbnails', -1, 'url'], # shows
     ))
 
     info['badges'] = []

youtube/yt_data_extract/everything_else.py

@@ -229,7 +229,7 @@ def extract_playlist_metadata(polymer_json):
     if metadata['first_video_id'] is None:
         metadata['thumbnail'] = None
     else:
-        metadata['thumbnail'] = f"https://i.ytimg.com/vi/{metadata['first_video_id']}/hq720.jpg"
+        metadata['thumbnail'] = f"https://i.ytimg.com/vi/{metadata['first_video_id']}/hqdefault.jpg"
 
     metadata['video_count'] = extract_int(header.get('numVideosText'))
     metadata['description'] = extract_str(header.get('descriptionText'), default='')