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/app/reader/queries.py

@@ -605,7 +605,9 @@ def takeout_lengths_to_ask(db: Session, user: User) -> list[str]:
 
 def watched_channels_pending(db: Session, user: User) -> int:
     """How many channels a later run may still add to the list: the candidates with a watched video
-    whose length is not known, and the ones that made it but the catalog cannot name yet."""
+    whose length is not known, and the ones that made it but have no catalog row to be listed from
+    yet. One the catalog holds without details is listed already (by the name it came in with), so
+    it is not one to come."""
     cand = _candidate_channels(user)
     unasked = db.execute(
         select(TakeoutItem.channel_id)
@@ -613,19 +615,30 @@ def watched_channels_pending(db: Session, user: User) -> int:
         .where(_unfollowed(user), TakeoutItem.looked_up_at.is_(None))
         .distinct()
     ).scalars()
-    return len(set(unasked) | set(watched_channels_to_describe(db, user)))
+    counted = _counted_channels(user)
+    unlisted = db.execute(
+        select(counted.c.channel_id)
+        .outerjoin(Channel, Channel.id == counted.c.channel_id)
+        .where(Channel.id.is_(None))
+    ).scalars()
+    return len(set(unasked) | set(unlisted))
 
 
-def watched_channels_to_describe(db: Session, user: User) -> list[str]:
-    """The channels on the list (by their counted videos) that the catalog lacks or has no details
-    of yet: what the list cannot name."""
-    counted = (
+def _counted_channels(user: User):
+    """The channels that make the list by their counted videos, whatever the catalog holds of them."""
+    return (
         select(TakeoutItem.channel_id)
         .where(_unfollowed(user), TakeoutItem.duration_seconds >= WATCHED_CHANNEL_MIN_SECONDS)
         .group_by(TakeoutItem.channel_id)
         .having(func.count() >= WATCHED_CHANNEL_MIN_VIDEOS)
         .subquery()
     )
+
+
+def watched_channels_to_describe(db: Session, user: User) -> list[str]:
+    """The channels on the list (by their counted videos) that the catalog lacks or has no details
+    of yet: what the list cannot name, or names with no more than a video brought."""
+    counted = _counted_channels(user)
     return list(
         db.execute(
             select(counted.c.channel_id)