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/routes/feed.py

@@ -481,6 +481,8 @@ def watched_channel_videos(
                 ensure_videos(db, yt, missing, explore=channel.from_explore)
         except YouTubeError as exc:
             raise youtube_refusal(exc) from exc
+        catalog_service.settle_explored(db, channel)
+        db.commit()
     items = queries.feed_rows_by_ids(db, user, ids)
     watched_at = dict(counted)
     for item in items: