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/subscriptions.py

@@ -1089,12 +1089,26 @@ def serve_subscription_thumbnail(thumbnail):
             f.close()
             return flask.Response(image, mimetype='image/jpeg')
 
-    url = f"https://i.ytimg.com/vi/{video_id}/hq720.jpg"
-    try:
-        image = util.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))
-        flask.abort(e.code)
+    image = None
+    for quality in ('hq720.jpg', 'sddefault.jpg', 'hqdefault.jpg'):
+        url = f"https://i.ytimg.com/vi/{video_id}/{quality}"
+        try:
+            image = util.fetch_url(url, report_text="Saved thumbnail: " + video_id)
+            break
+        except util.FetchError as e:
+            if '404' in str(e):
+                continue
+            print("Failed to download thumbnail for " + video_id + ": " + str(e))
+            flask.abort(500)
+        except urllib.error.HTTPError as e:
+            if e.code == 404:
+                continue
+            print("Failed to download thumbnail for " + video_id + ": " + str(e))
+            flask.abort(e.code)
+
+    if image is None:
+        flask.abort(404)
+
     try:
         f = open(thumbnail_path, 'wb')
     except FileNotFoundError: