Fix review findings and remove Uncategorized from sidebar

Backend: cache Google access tokens (drop dead access_token_expires_at,
migration 0007), handle MeTube cleared/canceled events by URL, return
email from /auth/status only when authenticated, move Google base URLs
into settings, run container as non-root.

Frontend: include local feed filters in the query key, remove dead
Saved page and unused assets, drop stale CategoryNav props and classes,
send Content-Type only with a body, remove Uncategorized from sidebar.

backend/app/services/sync.py

@@ -195,7 +195,7 @@ def sync_videos(db: Session) -> dict:
                 continue
 
             duration_seconds = parse_iso8601_duration(item["duration_iso8601"])
-            youtube_url = f"https://www.youtube.com/watch?v={item['youtube_video_id']}"
+            youtube_url = settings.youtube_watch_url_template.format(video_id=item["youtube_video_id"])
 
             video = existing.get(item["youtube_video_id"])
             if video is None: