fix(catalog): a follow from YouTube keeps an explored channel, and the list counts only what can come

A subscription pulled in from YouTube left an explored channel flagged as an exploration's, so its
later videos came in hidden from the Library with their watched marks waiting for good; a subscribe
landing while an exploration's videos came in left those flagged too. One step, settle_explored,
now holds the rule by who follows the channel now: the subscribe here, the subscription import, the
watched-channel videos and the explore page all end with it. Migration 0116 clears what the gap
left behind.

The list's "may still grow" count no longer counts a channel it already shows from a stub row, and
the comments on the cleanup's keep rule and on the in-memory daily shares say what they do.

backend/tests/test_watched_channels.py

@@ -10,6 +10,7 @@ from sqlalchemy import select, update
 from app import quota
 from app.catalog import service as catalog_service
 from app.catalog.models import BlockedChannel, Channel, ExploredChannel, Subscription, Video
+from app.db import SessionLocal
 from app.reader import queries, service, takeout
 from app.reader.models import TakeoutChannelDismissal, TakeoutItem, TakeoutKind
 from app.routes import feed as feed_routes
@@ -318,6 +319,18 @@ def test_naming_the_channels_is_tried_again_and_meanwhile_the_list_says_it_is_no
     assert queries.watched_channels_pending(db, user) == 0
 
 
+def test_a_listed_channel_still_to_be_described_is_not_counted_as_one_to_come(db, make_user):
+    user = make_user("stub@example.com")
+    # A stub the catalog made when one of its videos came in: a title, no details yet.
+    db.add(Channel(id=_ch(1), title="From a video"))
+    db.commit()
+    _watched(db, user, 1, [LONG] * 5)
+    assert queries.watched_channels_to_describe(db, user) == [_ch(1)]
+    assert _listed(db, user) == [(_ch(1), 5)]
+    # It is on the list already: "the list may grow" would promise one more that cannot come.
+    assert queries.watched_channels_pending(db, user) == 0
+
+
 def test_the_lengths_leave_the_watch_later_lookups_their_own_share(
     db, make_user, youtube, monkeypatch
 ):
@@ -400,6 +413,31 @@ def test_a_followed_channels_watched_videos_come_in_as_the_catalogs_own(
     assert db.get(Channel, _ch(1)).from_explore is False
 
 
+def test_a_follow_landing_while_the_videos_come_in_leaves_none_of_them_flagged(
+    db, make_user, as_user, monkeypatch
+):
+    user, follower = make_user("racer@example.com"), make_user("late@example.com")
+    _channel(db, 1)
+    _watched(db, user, 1, [LONG] * 5)
+    fake = _fake_ensure(calls := [])
+
+    def ensure(session, yt, ids, *, explore=False):
+        # Someone subscribes, in a request of their own, while YouTube answers this one.
+        with SessionLocal() as other:
+            ch = other.get(Channel, _ch(1))
+            catalog_service.record_subscription(other, follower, ch, "sub-late")
+            other.commit()
+        fake(session, yt, ids, explore=explore)
+
+    monkeypatch.setattr(feed_routes, "ensure_videos", ensure)
+    monkeypatch.setattr(feed_routes.quota, "backfill_reserve_reached", lambda: False)
+    assert as_user(user).post(f"/api/takeout/channels/{_ch(1)}/videos").status_code == 200
+    assert calls[0][1] is True  # asked as the exploration's
+    db.expire_all()
+    flagged = db.scalars(select(Video.id).where(Video.via_explore.is_(True))).all()
+    assert (db.get(Channel, _ch(1)).from_explore, flagged) == (False, [])
+
+
 def test_the_cleanup_keeps_a_channel_a_watch_history_lists_and_reclaims_its_videos(db, make_user):
     user = make_user("kept@example.com")
     _channel(db, 1)