When users filter channels by group without entering a search keyword,
the frontend calls GetAllChannels (GET /api/channel/) instead of
SearchChannels. However, GetAllChannels did not process the group
query parameter, causing the filter to have no effect.
Added group filtering logic to GetAllChannels for both normal mode
and tag mode, using the same CONCAT/|| pattern as SearchChannels
for cross-database compatibility (MySQL, PostgreSQL, SQLite).
