feat(channels): an Added column, and About cards that wait for a resting pointer

The channel manager lists when each follow reached the list (the
subscription import that first saw it, or a subscribe made here) as a
sortable Added date after Last upload. The About cell's card opens after the
pointer rests on it for half a second instead of at once, and a scroll puts
an open one away.

backend/app/routes/channels.py

@@ -76,6 +76,10 @@ def list_channels(user: User = Depends(current_user), db: Session = Depends(get_
             "details_synced": ch.details_synced_at is not None,
             "recent_synced": ch.recent_synced_at is not None,
             "backfill_done": ch.backfill_done,
+            # When the follow reached this list: written by the subscription import the run that
+            # first saw it, or by a subscribe made here — not the channel row's own date, which an
+            # explored or discovered channel had before it was followed.
+            "added_at": iso(sub.created_at),
         }
         for sub, ch in rows
     ]