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/util.py

@@ -542,21 +542,31 @@ class RateLimitedQueue(gevent.queue.Queue):
 
 
 def download_thumbnail(save_directory, video_id):
-    url = f"https://i.ytimg.com/vi/{video_id}/hq720.jpg"
     save_location = os.path.join(save_directory, video_id + ".jpg")
-    try:
-        thumbnail = fetch_url(url, report_text="Saved thumbnail: " + video_id)
-    except urllib.error.HTTPError as e:
-        print("Failed to download thumbnail for " + video_id + ": " + str(e))
-        return False
-    try:
-        f = open(save_location, 'wb')
-    except FileNotFoundError:
-        os.makedirs(save_directory, exist_ok=True)
-        f = open(save_location, 'wb')
-    f.write(thumbnail)
-    f.close()
-    return True
+    for quality in ('hq720.jpg', 'sddefault.jpg', 'hqdefault.jpg'):
+        url = f"https://i.ytimg.com/vi/{video_id}/{quality}"
+        try:
+            thumbnail = fetch_url(url, report_text="Saved thumbnail: " + video_id)
+        except FetchError as e:
+            if '404' in str(e):
+                continue
+            print("Failed to download thumbnail for " + video_id + ": " + str(e))
+            return False
+        except urllib.error.HTTPError as e:
+            if e.code == 404:
+                continue
+            print("Failed to download thumbnail for " + video_id + ": " + str(e))
+            return False
+        try:
+            f = open(save_location, 'wb')
+        except FileNotFoundError:
+            os.makedirs(save_directory, exist_ok=True)
+            f = open(save_location, 'wb')
+        f.write(thumbnail)
+        f.close()
+        return True
+    print("No thumbnail available for " + video_id)
+    return False
 
 
 def download_thumbnails(save_directory, ids):