channels_with_threads opened a fresh SQLite connection per
release-channel database on every call, and the sidebar called it twice
per frame from its render path, blocking the render loop on disk I/O.

Run the per-channel checks on a background thread and cache the
resulting channel names once at sidebar construction, since the
per-channel databases cannot change while this process runs.

Release Notes:

- Fixed a cause of stutters in the agent panel sidebar rendering