feat(managers): About column with hover overlay on both channel tables (E4 S2)

- Backend: add channel description to the shared _channel_summary projection, so both
  the subscribed list and the discovery list carry it (no migration; description is
  already stored from the channels.list call)
- New AboutCell: one truncated line in the table + full text in a portalled, always-on
  glass hover/focus overlay (wider than Tooltip, flips above the anchor near the bottom,
  closes on scroll/resize)
- About column (after Channel) on both the subscribed and discovery tables; hidden in the
  mobile card fallback

frontend/src/lib/api.ts

@@ -510,6 +510,7 @@ export interface ManagedChannel {
   thumbnail_url: string | null;
   subscriber_count: number | null;
   video_count: number | null;
+  description: string | null;
   stored_videos: number;
   last_video_at: string | null;
   total_duration_seconds: number;
@@ -535,6 +536,7 @@ export interface DiscoveredChannel {
   thumbnail_url: string | null;
   subscriber_count: number | null;
   video_count: number | null;
+  description: string | null;
   playlist_video_count: number; // how many of the user's playlist videos are from here
   playlist_count: number; // across how many of their playlists
   details_synced: boolean;