feat(channels): aggregate columns + smarter tag display

- Add Last upload, total Length and a Normal/Shorts/Live breakdown, computed on read in the
  existing grouped query (~35ms over the full catalog — measured, so no denormalization).
- Subscribers shown compact (919K), since the YouTube API already rounds them.
- Tag cell now shows only the tags actually attached to a channel; a '+' opens a per-channel
  picker to attach/detach, instead of listing every user tag on every row.

frontend/src/lib/api.ts

@@ -293,6 +293,11 @@ export interface ManagedChannel {
   subscriber_count: number | null;
   video_count: number | null;
   stored_videos: number;
+  last_video_at: string | null;
+  total_duration_seconds: number;
+  count_normal: number;
+  count_short: number;
+  count_live: number;
   priority: number;
   hidden: boolean;
   deep_requested: boolean;