fix(channels): a channel YouTube no longer returns is not asked for on every view

channels.list simply omits a terminated channel, so nothing was stamped and the
channel page spent a quota unit on it every time. The miss is stamped like a
fetch; the published_at sentinel that kept it stale is gone.

backend/tests/test_channel_details_refresh.py

@@ -28,7 +28,7 @@ def _item(cid: str) -> dict:
     }
 
 
-def _youtube(asked: list[list[str]], subscribed: list[str]):
+def _youtube(asked: list[list[str]], subscribed: list[str], gone: tuple[str, ...] = ()):
     class _Client:
         def __init__(self, db, user):
             pass
@@ -45,7 +45,8 @@ def _youtube(asked: list[list[str]], subscribed: list[str]):
 
         def get_channels(self, ids):
             asked.append(sorted(ids))
-            return [_item(cid) for cid in ids]
+            # A terminated / deleted channel is simply absent from `channels.list`'s answer.
+            return [_item(cid) for cid in ids if cid not in gone]
 
     return _Client
 
@@ -105,3 +106,38 @@ def test_the_channel_page_refreshes_a_stale_channel_only(
     assert asked == ([["UCpage"]] if refreshed else [])
     suffix = "new" if refreshed else "old"
     assert body["banner_url"] == f"https://yt3.example/UCpage-{suffix}"
+
+
+def test_a_channel_youtube_no_longer_returns_is_not_asked_for_on_every_view(
+    db, make_user, monkeypatch
+):
+    """A terminated channel is missing from `channels.list`'s answer, so nothing was stamped and the
+    channel page asked for it again (1 quota unit) on every single view. The miss is stamped like a
+    fetch, so the next ask waits for `DETAILS_MAX_AGE` like any other channel."""
+    user = make_user()
+    db.add(Channel(id="UCgone", title="UCgone"))
+    db.commit()
+    asked: list[list[str]] = []
+    monkeypatch.setattr(channels, "YouTubeClient", _youtube(asked, [], gone=("UCgone",)))
+
+    channels.channel_detail("UCgone", user=user, db=db)
+    channels.channel_detail("UCgone", user=user, db=db)
+
+    assert asked == [["UCgone"]]
+
+
+def test_the_import_stamps_a_channel_youtube_no_longer_returns(db, make_user, monkeypatch):
+    user = make_user()
+    _channel(db, "UCgone", STALE)
+    db.flush()
+    db.add(Subscription(user_id=user.id, channel_id="UCgone"))
+    db.commit()
+    asked: list[list[str]] = []
+    monkeypatch.setattr(
+        subscriptions, "YouTubeClient", _youtube(asked, ["UCgone"], gone=("UCgone",))
+    )
+
+    import_subscriptions(db, user)
+    import_subscriptions(db, user)
+
+    assert asked == [["UCgone"]]