feat(channels): surface full-history pull + honest stored/total count on the channel page (S3)

The catalog only stores the recent-backfill/RSS subset of a channel's uploads (video_count is
YouTube's raw total), so a subscribed channel looked like it was "missing" videos. Now the channel
page header shows "{stored} / {total} videos" when there's a gap, and a "Full history" action queues
the deep backfill (the same deep_requested trigger the channel manager uses) — showing a loading
state until done. Backend: channel_detail now returns deep_requested/backfill_done.

frontend/src/lib/api.ts

@@ -136,6 +136,8 @@ export interface ChannelDetail {
   explored: boolean;
   blocked: boolean;
   stored_videos: number;
+  deep_requested: boolean;
+  backfill_done: boolean;
   from_explore: boolean;
   details_synced: boolean;
 }