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/sync/subscriptions.py

@@ -8,6 +8,7 @@ from sqlalchemy.orm import Session
 
 from app import logs
 from app.models import User
+from app.catalog import service as catalog_service
 from app.catalog.models import Channel, Subscription
 from app.utils import normalize_description
 from app.youtube.client import YouTubeClient, YouTubeError, best_thumbnail
@@ -197,6 +198,10 @@ def import_subscriptions(db: Session, user: User) -> dict:
                 db.add(sub_row)
             sub_row.yt_subscription_id = sub.get("yt_subscription_id")
         db.flush()
+        # A follow made on youtube.com keeps an explored channel as a follow made here does.
+        explored = select(Channel).where(Channel.id.in_(fetched), Channel.from_explore.is_(True))
+        for channel in db.execute(explored).scalars():
+            catalog_service.settle_explored(db, channel)
 
         # Prune subscriptions removed on YouTube — but only when this fetch is trustworthy
         # (see `should_prune`).