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
     ]

backend/tests/test_catalog_routes.py

@@ -72,3 +72,23 @@ def test_the_sync_status_counts_the_callers_subscriptions(db, make_user, as_user
     assert (body["subscriptions"], body["channels_total"]) == (1, 1)
     mine = as_user(user).get("/api/sync/my-status").json()
     assert (mine["channels_total"], mine["channels_recent_pending"]) == (1, 1)
+
+
+def test_a_channel_lists_when_it_entered_this_users_list_not_when_the_channel_did(
+    db, make_user, as_user
+):
+    from datetime import datetime, timezone
+
+    user = make_user()
+    db.add(Channel(id="UCa", title="A", created_at=datetime(2026, 6, 10, tzinfo=timezone.utc)))
+    db.flush()
+    db.add(
+        Subscription(
+            user_id=user.id,
+            channel_id="UCa",
+            created_at=datetime(2026, 10, 12, 8, 30, tzinfo=timezone.utc),
+        )
+    )
+    db.commit()
+    listed = as_user(user).get("/api/channels").json()
+    assert [(c["id"], c["added_at"]) for c in listed] == [("UCa", "2026-10-12T08:30:00+00:00")]